PDS3 encoding options

[rbeyer]

Description

Added optional parameters (with specification-appropriate defaults) to pvl.encoder.PDSLabelEncoder().__init__() which allow users to optionally not write the trailing "Z" on time and datetime output, and also allow users to optionally make PVL Symbol Strings handled the same as PVL Text Strings (instead of handling them separately and enclosing them in single-quotes).

Related Issue

This would close #84.

How Has This Been Tested?

• make lint
• make docs
• make test-all

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• 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.