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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.82%. Comparing base (1e0dca4) to head (acd7a18).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   77.70%   77.82%   +0.11%     
==========================================
  Files          10       10              
  Lines        1310     1308       -2     
==========================================
  Hits         1018     1018              
+ Misses        292      290       -2     

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[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 🙏