C xdi_reader passing tests on baddata that should give useful message

[newville]

Including:

inconsistent number of columns
NaN in data
strings in data

and so on.

Currently missing '# Column.N ' lines are silently ignored, replaced by a label of 'colN', and too many column labels is also silently ignored. No opinion on correct behavior -- could simply document this tolerant behavior.

[bruceravel]

I noticed this this week, then came across this issue. Given that xdifile is returning positive error codes as warnings, along with error messages, it seems appropriate to return a warning/comment for each of those situations.

I didn't include this in today's PR, but will implement it later.