Add test for ElementLineHermite

[ahojukka5]

Add test to make sure that exceptions are thrown when calling gdof with index > 3. After merging this PR, file element_line_hermite.py is having 100 % test coverage.

[ahojukka5]

Internally, gdof is calling index_error which raises ValueError. Is this intentional?

[kinnala]

Internally, gdof is calling index_error which raises ValueError. Is this intentional?

Maybe IndexError would be more appropriate?

exception ValueError
Raised when an operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError.

exception IndexError
Raised when a sequence subscript is out of range.

[kinnala]

Note: Apparently the flake8 check I added using Github Actions yesterday is not running on pull requests. I need to look what's causing it.

[kinnala]

Now it runs but there seems to be an issue with the Github Action I used: wearerequired/lint-action#13