Deprecate type=calculate and type=hidden

[MartijnR]

Tbc, after discussion in pyxform: XLSForm/pyxform#508

If agreed:

• mention that type=hidden and type=calculate are exactly the same as type=text and should be considered deprecated
• in all examples, replace type=calculate and type=hidden with type=text (or replace type=calculate with type=hidden, depending on outcome of discussion)
• edit the calculate section (and any section that mentions calculate/calculation) if necessary

[lognaturel]

I'm happy to deprecate hidden but for once I disagree with @MartijnR and am pretty strongly against deprecating calculate. More at XLSForm/pyxform#508 (comment).