Fixes to make tests pass with Pegasus event files only directory.

[PeterNSteinmetz]

This set of changes fixes a bug which prevented the Pegasus .Nev event only files from passing the test suite.

It also allows filename extensions to be in any letter case. For the InputInverted header parameter to be missing,
meaning not inverted. Checks for 8 hashes at the start of the header as a check on header contents. Allows
any whitespace between a header key and value.

Internal changes are addition of a large number of explanatory comments and refactoring of the code to handle
the header into a separate class. Many of these in preparation for handling earlier versions of neuralynx file headers
and more flexible handling of segments withins .Ncs files.

[pep8speaks]

Hello @PeterNSteinmetz! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-10-28 15:42:23 UTC

[apdavison]

Many thanks for this PR. Please also add your name and affiliation to doc/source/authors.rst

[samuelgarcia]

Hi Peter,
thank you very much for this.
Except the comment I have made in the code, I think I am OK with this patch.
Lets see if Andrew or Julia will have time to review this.
Cheers
Samuel

[PeterNSteinmetz]

It is definitely easy to revert the change of the local argument name to channel_indices if that is what people want. Since this is a name simply local to the function and makes no difference to the caller, I would suggest just gradually changing this spelling in functions as they are changed for other reasons.

[PeterNSteinmetz]

Bumping this. Can I get some advice on use of channel_indices for local argument names in non-public functions, i.e., those beginning with _ ? I can then change this in both of my current pull requests.

[JuliaSprenger]

Hi @PeterNSteinmetz, thanks for the the PR . Look good to me. Please just revert the name changes for channel_indices and I would be happy to merge.

[JuliaSprenger]

There are still a number of TODO notes in your changes. Are you planning to address these in this PR, i.e. is this PR still work in progress?

[PeterNSteinmetz]

Appears now to past all tests and should be ready to merge.