Streamline internal de/conditioning interface

[penelopeysm]

This PR:

• in src/contexts.jl, removes the methods AbstractPPL.condition(values, ...) in favour of directly using the ConditionContext() constructor.

  This is mainly inspired by Aqua's complaints about how we are pirating AbstractPPL.{de,}condition (see Add Aqua tests #775). I've tested locally and this PR would fix all of the condition-related type piracies. But also semantically, I think that condition is something you do to a model, not a context (and AbstractPPL's API sort of specifies that too). Since contexts are DynamicPPL-internal, I think it makes sense that we use an different, internal, function to manipulate that.

• likewise, renames AbstractPPL.decondition(context, ...) to decondition_context(context, ...). Suggestions for other names welcome!

• shifts the bulk of the 'figuring-out-arguments-and-coercing-to-the-right-format' stuff into a helper function, called _make_conditioning_values. This allows us to have a much more specific type for the ConditionContext struct itself, while retaining the current flexibility of the model conditioning syntax.

• adds tests for all of the above

---

Going beyond the name changes, there are two changes to functionality, which are both in the new decondition_context:

• decondition_context now checks if there are no more conditioned values left, and if so, unwraps the layer of ConditionContext. This shouldn't really affect anything, it just makes for cleaner context trees. For example:

  julia> ctx = ConditionContext(Dict(@varname(x) => 1, @varname(y) => 2))
  ConditionContext(Dict{VarName{sym, typeof(identity)} where sym, Int64}(x => 1, y => 2), DefaultContext())
  
  julia> # On current master
         decondition(ctx, @varname(x), @varname(y)) # On current master
  ConditionContext(Dict{VarName{sym, typeof(identity)} where sym, Int64}(), DefaultContext())
  
  julia> # With this PR
         decondition_context(ctx, @varname(x), @varname(y))
  DefaultContext()

• decondition_context also makes sure to deepcopy the previous conditioned values, to avoid surprises due to mutation in BangBang.delete!!:

  julia> ctx = ConditionContext(Dict(@varname(x) => 1, @varname(y) => 2))
  ConditionContext(Dict{VarName{sym, typeof(identity)} where sym, Int64}(x => 1, y => 2), DefaultContext())
  
  julia> # On current master
         decondition(ctx, @varname(x)); decondition(ctx, @varname(y))
  ConditionContext(Dict{VarName{sym, typeof(identity)} where sym, Int64}(), DefaultContext())
  
  julia> # With this PR
         decondition_context(ctx, @varname(x)); decondition_context(ctx, @varname(y))
  ConditionContext(Dict{VarName{sym, typeof(identity)} where sym, Int64}(x => 1), DefaultContext())

[coveralls]

Pull Request Test Coverage Report for Build 12700023444

Details

• 14 of 32 (43.75%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 86.345%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/model.jl	9	10	90.0%
src/contexts.jl	5	22	22.73%

Files with Coverage Reduction	New Missed Lines	%
src/contexts.jl	2	30.16%

Totals
Change from base Build 12657521724:	0.1%
Covered Lines:	3737
Relevant Lines:	4328

---

💛 - Coveralls

[sunxd3]

This makes a ton of sense to me, I really like it.
Just some nitpicking, really.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.26%. Comparing base (003ff2f) to head (2597193).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/contexts.jl	86.36%	3 Missing ⚠️
src/model.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #776      +/-   ##
==========================================
+ Coverage   86.13%   86.26%   +0.12%     
==========================================
  Files          36       36              
  Lines        4336     4332       -4     
==========================================
+ Hits         3735     3737       +2     
+ Misses        601      595       -6     

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[sunxd3]

Thanks Penny, no more issue from my end