Update taxon builder tools

[davidorme]

This PR fixes #106

• The tools now take an explicit file argument rather than taking a directory and using an internally generated filename.
• The parser arguments are checked to make sure the path is available.
• The underlying taxondb functions are broken up a bit to separate the timestamp validation into standalone functions that are now tested.
• The GBIF build is updated to handle absurdly long text fields in the GBIF backbone dumps.

[codecov-commenter]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (177abd5) 78.11% compared to head (80dfec4) 79.06%.

Files	Patch %	Lines
safedata_validator/taxondb.py	64.81%	19 Missing :warning:
safedata_validator/entry_points.py	92.85%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #115 +/- ## =========================================== + Coverage 78.11% 79.06% +0.95% =========================================== Files 13 13 Lines 3674 3712 +38 =========================================== + Hits 2870 2935 +65 + Misses 804 777 -27 ```

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[davidorme]

Note that the new tests do actually call on remote resources (GBIF and NCBI) but only for small payloads to validate timestamps. This links into #57, so that when this gets tackled, these tests should also be mockable.