Allowing for unquoted plus signs for ISIS output and improves some default behavior

[rbeyer]

Description

• ISIS creates PVL text with unquoted plus signs ("+"), needed to adjust the ISISGrammar and OmniGrammar objects to parse this properly (#59).
• In the process of doing so, realized that we have some classes that optionally take a grammar and a decoder, and if they aren't given, to default. However, a decoder has a grammar object, so if a grammar isn't provided, but a decoder is, the grammar should be taken from the decoder, otherwise you could get confusing behavior.
• Updated pvl_validate to be explicit about these arguments.
• Added a --version argument to both pvl_translate and pvl_validate.

Motivation and Context

Addresses and would close #59 and a little bit more. Also exercises the fancy new PR template.

How Has This Been Tested?

• make lint
• make test-all
• make docs

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• I have updated the documentation accordingly.
• 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.