Fix: Restore extensibility of ProfileParser and use load_data from diffpy.utils

[cadenmyers13]

closes #194

This PR reworks ProfileParser so metadata and data parsing are extensible per format:

• ProfileParser.parse_file is now a template method backed by _parse_metadata/_parse_data. If a user wants to create their own parser, they have the option to parse data and metadata separately with the respective private functions.
• PDFParser is now identical to ProfileParser which uses load_data. I think this is okay for now, but we might want to reconsider in the future.
• parse_file now accepts load_data's keyword arguments (usecols, delimiter, comments, etc.), so files with more than four columns can be read by selecting columns explicitly.
• PDFContribution.loadData now takes a file name only (string/open-file input has been removed, consistent with the rest of the parser).
• parseString is removed from ProfileParser and PDFParser, since parsers now read from a file.
• The si-q27r60-xray.gr and ni-q27r100-neutron.gr test files were updated from 2008-era PDFgetX2/PDFgetN headers to modern diffpy.pdfgetx/xPDFsuite header formats (data values unchanged), and examples/docs were updated to use the new parser.

[cadenmyers13]

I reworked this old test to be up to our standards. See below

[codecov]

Codecov Report

❌ Patch coverage is 99.26471% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.89%. Comparing base (0b16bfd) to head (cd002d3).
⚠️ Report is 5 commits behind head on v3.3.0.

Files with missing lines	Patch %	Lines
tests/test_sas.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           v3.3.0     #196      +/-   ##
==========================================
+ Coverage   79.08%   81.89%   +2.80%     
==========================================
  Files          25       25              
  Lines        3821     3927     +106     
==========================================
+ Hits         3022     3216     +194     
+ Misses        799      711      -88     

Files with missing lines	Coverage Δ
tests/test_fitrecipe.py	99.85% <100.00%> (+<0.01%)	⬆️
tests/test_pdf.py	54.28% <100.00%> (+2.11%)	⬆️
tests/test_profileparser.py	100.00% <100.00%> (ø)
tests/test_sas.py	15.78% <0.00%> (ø)

... and 2 files with indirect coverage changes

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[cadenmyers13]

@sbillinge ready for review. See the news file and my above message for a good summary of whats been changed