add size for column of Map

[ayoub-benali]

The curent implementation of size doesn't handle Map as it needs two types for key and value.

The following example leads to this error:

    import frameless.TypedDataset
    import frameless.functions.size
    case class Foo(a: Map[String, String])
    val ds = TypedDataset.create(List(Foo(Map("foo" -> "bar"))))
    ds.select(size(ds('a)))

no type parameters for method size: (column: frameless.TypedColumn[T,V[A]])(implicit evidence$1: frameless.functions.CatalystSizableCollection[V])frameless.TypedColumn[T,Int] exist so that it can be applied to arguments (frameless.TypedColumn[Foo,Map[String,String]])
[error]  --- because ---
[error] argument expression's type is not compatible with formal parameter type;
[error]  found   : frameless.TypedColumn[Foo,Map[String,String]]
[error]  required: frameless.TypedColumn[?T,?V[?A]]
[error]     ds.select(size(ds('a)))

This PR adds a size function to handle Map

[codecov-io]

Codecov Report

Merging #255 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
+ Coverage   96.56%   96.57%   +<.01%     
==========================================
  Files          51       51              
  Lines         874      875       +1     
  Branches       11       11              
==========================================
+ Hits          844      845       +1     
  Misses         30       30

Impacted Files	Coverage Δ
...ain/scala/frameless/functions/UnaryFunctions.scala	100% <100%> (ø)	⬆️

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[OlivierBlanvillain]

LGTM, thanks!

[imarios]

@ayoub-benali We already had a typeclass CatalystSizableCollection that does that. Did you try adding Map there and it didn't work?

[imarios]

hmm, maybe the fact that Map has two holes and all the other ones have one hole might have caused an issue. was that the case? It would work with any V[_] but maybe not with V[_,_]?