Fixing new.py, lots of other minor fixes, and lots of tests.

[rbeyer]

• Attempting to import pvl.new without multidict being available, will now properly yield an ImportError.
• The dump() and dumps() functions now properly overwritten in pvl.new.
• All encoders that descended from PVLEncoder didn't properly have group_class and object_class arguments to their constructors, now they do.
• The char_allowed() function in grammar objects now raises a more useful ValueError than just a generic Exception.
• The new collections.PVLMultiDict wasn't correctly inserting Mapping objects with the insert_before() and insert_after() methods.
• The token.Token class's __index__() function didn't always properly return an index.
• The token.Token class's __float__() function would return int objects if the token could be converted to int. Now always returns floats.
• So many tests, increased coverage by a lot.

Related Issue

Would close #89

How Has This Been Tested?

• make lint
• make docs
• make test-all

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Licensing:

This project is released under the LICENSE.

• I claim copyrights on my contributions in this pull request, and I provide those contributions via this pull request under the same license terms that the pvl project uses.

[codecov]

Codecov Report

Merging #90 (fa6f196) into main (4c4f691) will increase coverage by 14.99%.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##             main      #90       +/-   ##
===========================================
+ Coverage   79.85%   94.84%   +14.99%     
===========================================
  Files          12       12               
  Lines        1911     1922       +11     
===========================================
+ Hits         1526     1823      +297     
+ Misses        385       99      -286     

Impacted Files	Coverage Δ
pvl/encoder.py	89.24% <ø> (+1.22%)	⬆️
pvl/new.py	86.00% <83.33%> (+86.00%)	⬆️
pvl/__init__.py	94.73% <100.00%> (ø)
pvl/collections.py	98.70% <100.00%> (+27.67%)	⬆️
pvl/grammar.py	100.00% <100.00%> (+3.88%)	⬆️
pvl/pvl_translate.py	100.00% <100.00%> (+100.00%)	⬆️
pvl/pvl_validate.py	99.06% <100.00%> (+99.06%)	⬆️
pvl/token.py	100.00% <100.00%> (+10.71%)	⬆️
pvl/lexer.py	100.00% <0.00%> (+0.76%)	⬆️
... and 9 more

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