Deprecation of array indexing calls `[]`

[chiyahn]

As suggested in #206 (comment), I propose the following procedural changes to deprecate interpolation evaluation by array indexing []:

• Support interpolation evaluation by function calls () (complete from Function call syntax support #206)
• ModifyREADME.md to let users know that [] is deprecated in favour of () (complete from Function call syntax support #206)
• Start throwing deprecation warnings for evaluation with []
• Modify unit tests and files in doc directory that evaluate interpolation objects by [] to ()
• Modify any script in package that evaluates interpolation objects by [] to ()
• (After tagging a major release) remove [] from the code base entirely

The first two steps will be completed as soon as the pull request #206 is approved and changes are committed accordingly. Completing the rest of the procedure should not be a complex task.

[tomasaschan]

Nice! I suggest adding to the to do-list also the following actions:

• start throwing depreciation warnings for evaluation with []
• (after tagging a major release) remove [] from the code base entirely

[chiyahn]

No problem, just added them to the list!

[lstagner]

Instead of removing [] syntax entirely would it be possible to just have it used for accessing the underlying value array i.e. itp[i] == itp(x[i])