GH: 3156 caching bugs

[alanakbik]

Draft PR that removes custom equality on DataPoint and increases cache size for context computation. More testing needed if the first is possible without breaking other parts of the code.

[alanakbik]

Update: to fix bug #3157, the following things were changed:

• the custom equality definition from DataPoint was removed. Two data points are now only the same if they are the same object (previously two data points were the same if they had the same text)
• the internal handling of how _PartOfSentence objects (like Span) are created is changed to avoid duplicate object creation
• the caching was removed altogether. I tried different caching sizes and there seems to be no impact on overall speeds.