Refactor date.py

[limlam96]

What?
This collection of transformers needs a bit of a rework before setting up the new testing framework, the classes are set up inconsistently in a way that does not support the new approach.

Suggested actions:

• BaseDateTransformer to implement transform method, that calls check_columns_are_date_or_datetime
• Deprecate cast_columns_to_datetime from BaseDateTransformer, throw error directing to existing ToDatetimeTransformer if columns fail type checks
• Some complications are introduced by transformers checking for datetime OR date type, which allows the possibility of mismatched types across manipulated columns. Suggest we make transformers more opinionated, introduce a type argument that allows date or datetime and checks accordingly. This would allow us to deprecate '_cast_non_matching_columns', 'match_column_datatypes'

Other thoughts:

• We have DateDiffLeapYearTransformer and DateDifferenceTransformer, can these be condensed?

[davidhopkinson26]

Consolidating DateDiffLeapYearTransformer and DateDifferenceTransformer now has its own issue in #244

[limlam96]

Closed with this PR #246