Add typed calculates

The underlying data type of the XLSForm `calculate` type is `string`. `pyxform` now also supports using other types with a calculation.

Start by locating the XLSForm documentation for the calculate type and review how pyxform describes calculation data types. Update the documentation to explain that calculate fields can use types beyond string, and confirm the supported typed-calculation behavior is accurately represented.

Written by the indexing model from the issue text.