Add VideoIO.framerate(f) to the manual and expand tests

[IanButterworth]

Related to #392

Todo:

• Add expected values tests for example videos
• Add generated video tests with more fps values

cc. @AbelHo

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (755ad16) 79.43% compared to head (7f2c717) 79.43%.

❗ Current head 7f2c717 differs from pull request most recent head 7eb87fc. Consider uploading reports for the commit 7eb87fc to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #393   +/-   ##
=======================================
  Coverage   79.43%   79.43%           
=======================================
  Files          10       10           
  Lines        1279     1279           
=======================================
  Hits         1016     1016           
  Misses        263      263           

Impacted Files	Coverage Δ
src/avio.jl	76.49% <100.00%> (+0.10%)	⬆️

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[AbelHo]

results from this 7f2c717 :

julia> VideoIO.framerate.(VideoIO.openvideo.(fnames))
5-element Vector{Rational{Int32}}:
 1//90000
 1//12800
 1//15360
 1//1000
 1//25000

julia> map(fnames) do x
               vid=VideoIO.openvideo(x); read(vid); VideoIO.framerate(vid)
       end
5-element Vector{Rational{Int32}}:
 1//90000
 1//12800
 1//15360
 1//1000
 1//25000

julia> get_fps.(fnames)
5-element Vector{Rational{Int64}}:
 30//1
 25//1
 30//1
 25//1
 25//1

[IanButterworth]

@AbelHo @f3fora would you mind reviewing this. I can't see why the tests are failing

[IanButterworth]

Bump @AbelHo (I saw the thumbs up. Thanks!)

[IanButterworth]

Bump. Help would be appreciated 🙏