Fix broken HTML and XML local generation

[davidorme]

This PR fixes #109

The main changes are:

• Update the XML and HTML generation functions to work with the version 3.0.0 metadata inputs.
• Add some test files into the virtual filesystem to allow testing of those endpoints
• Update the endpoints to work correctly
• Add proper return codes to the entry points rather than returning None.

Some things to fix:

• How does the XML distribution work in version 3+.
  • We can now simply generate the XML and add it to the deposit before publishing.
  • We probably need backwards compatibility to link to the metadata server, but it would be good to keep open the option of publishing datasets without having to use the metadata server.

[codecov-commenter]

Codecov Report

Attention: 30 lines in your changes are missing coverage. Please review.

Comparison is base (39dadd8) 72.08% compared to head (78c545a) 78.11%. Report is 1 commits behind head on develop.

Files	Patch %	Lines
safedata_validator/entry_points.py	47.16%	28 Missing :warning:
safedata_validator/zenodo.py	93.33%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #111 +/- ## =========================================== + Coverage 72.08% 78.11% +6.03% =========================================== Files 13 13 Lines 3668 3674 +6 =========================================== + Hits 2644 2870 +226 + Misses 1024 804 -220 ```

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[davidorme]

This also does quite a bit of groundwork for #110