Fixing minor bugs, changing `scrape_namespaces` for timeout issue

[esimpsons3ti]

Addressed a few fixes:

1. If a user attempts to sort by a key that does not exist within the available headers, a new error message will now print:

   Unknown sort key 'pds:lohical_identifier'. For a list of available sort keys, use the --output-headers-file option.

2. scrape_namespaces has been changed to use the Element trees of the schema files, instead of loading in the schema files every time it is used. This previously called a timeout error.

3. Fixed an issue where empty classes in label files were mistaken for attributes. Since classes don't have data types, any classes that are caught this way will be given the inapplicable value for their data type in label generation.

Fixes #22

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 71.20%. Comparing base (908e16d) to head (f3cd677). Report is 2 commits behind head on main.

Files	Patch %	Lines
pds4indextools/pds4_create_xml_index.py	26.66%	10 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #26 +/- ## ========================================== - Coverage 71.23% 71.20% -0.03% ========================================== Files 1 1 Lines 591 587 -4 Branches 133 134 +1 ========================================== - Hits 421 418 -3 + Misses 146 144 -2 - Partials 24 25 +1 ```

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