Fllc interface and docs improvements

[mbkuhn]

Summary

Make sure that the error message is clear and helpful for when faulty fllc_type is provided. Clarify what the default value is in the documentation and fix format there.

Pull request type

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Checklist

The following is included:

• new unit-test(s)
• new regression test(s)
• documentation for new capability

This PR was tested by running:

• the unit tests
  • on GPU
  • on CPU
• the regression tests
  • on GPU
  • on CPU

Additional background

I checked the behavior of the new check using the act_fixed_wing_fllc regression test with different input lines.

Issue number: closes #1407

[mbkuhn]

@rthedin