Use a consistent return_type for Count* primitives

In some counting primitives we are using a `Double` return type (like `PartOfSpeechCount`) while in others `IntegerNullable` is used as the return type (like `NumUniqueSeparators`).

Given that all of the counting primitives should be returning a whole number (or optionally null), `IntegerNullable` seems like a more appropriate return type to use than `Double`. We should investigate if there is a reason for using `Double` in some primitives, and, if possible, convert these to use an `IntegerNullable` return type.

Primitives to study:
- MedianWordLength
- PartOfSpeechCount
- PunctuationCount
- TitleWordCount
- TotalWordLength
- UpperCaseCount