Properly handle unicode values passed to cmor.axis in Python

[mauzey1]

Fixes #612

When string values get passed to cmor.axis, they will be treated as a Numpy array of type numpy.unicode_.

From https://numpy.org/doc/stable/reference/arrays.dtypes.html

Note that str refers to either null terminated bytes or unicode strings depending on the Python version. In code targeting both Python 2 and 3 np.unicode_ should be used as a dtype for strings.

The C code has been modified to properly copy string values from a Numpy array to a C array to be processed by cmor_axis.

[durack1]

@mauzey1 I wonder if explicitly decoding to unicode e.g.

string.decode('utf-8')

Would be a good way to force all types?

[mauzey1]

@durack1
The current line below should already be performing this conversion.

coord_vals = numpy.array(coord_vals, numpy.unicode_)

I could make the conversion more explicit by using

if data_type == 'S':
    coord_vals = numpy.char.decode(coord_vals, encoding='utf-8')

[durack1]

@mauzey1 my comment was just that. I found that in generating html content from the contributed contents for the CMIP6_CVs all manner of weird non-standard (and non UTF-8) characters were sneaking in (presumably from folks copying characters out of Word, or some other rich text software), and the parsers weren't expecting these characters and consequently barfed.

I think this is true to CMOR, so catching non UTF-8 characters and throwing an explicit error (if a decode function can't handle things) would be my more bulletproof preference

[durack1]

@mauzey1 happy to green light your contribution, it looks good, but was just wanting to make the statement about non-UTF-8 chars for the record

[mauzey1]

@durack1 After some experimenting with unicode, I decided to change the code to explicitly convert the strings to UTF-8.

[durack1]

@mauzey1 I think that was a wise tweak, hopefully, it catches those fringe cases