Disallow logical propositions with only 1 logical variable

[hdavid16]

Currently, @constraint(m, y := true) throws an error, but @constraint(m, logical_not(y) := true does not. For consistency, both should error since the user should fix the value of the logical variable instead. This PR makes adds additional checks to enforce this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b26e1ed) 99.89% compared to head (79c54c0) 99.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #92   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files          10       10           
  Lines         963      967    +4     
=======================================
+ Hits          962      966    +4     
  Misses          1        1           

Files	Coverage Δ
src/constraints.jl	100.00% <100.00%> (ø)

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[pulsipher]

This is good. The only thing is that with these changes I think the following is unnecessary:

DisjunctiveProgramming.jl/src/logic.jl

Lines 275 to 276 in b26e1ed

	if _isa_literal(lexpr)
	add_to_expression!(func, 1 - _reformulate_clause(model, lexpr.args[1]))

. This is the line that is missing the code coverage.

[hdavid16]

It is still needed to reformulate:

@constraint(model, y[1] ∧ ¬y[2] := true)

I have updated test_intersection_reformulation() to use this constraint for the test, which should allow for testing the line you indicate.

[pulsipher]

Looks good