Issue_58 - Add support for conditions

[adahms]

Hey @Levi-Leah - could you review this one?

This PR adds supports for ifdef and ifndef conditions when parsing global attributes and coalescing files.

It also addresses the following edge cases -

• Merged the global attribute parsing function into the coalescing function so that the global attributes file is treated like any other modular documentation file. This has the added benefit of compiling attributes from includes in the global attributes file into a single dictionary, as in the RHDG documentation.
• Updates line tests to use the same regex format, and tests for attributes in includes that aren't at the start of the include.

[Levi-Leah]

@adahms I added a few suggestions regarding the simplification of some if statements, please take them with a grain of salt

[adahms]

Hey @Levi-Leah - finally returning to this one.

I was definitely tying myself up in knots a bit there - I've accepted several of your suggestions and re-worked the logic so that it's a little easier to read and parse.

Let me know what you think if you've got the time for a review. :)

[Levi-Leah]

I'll try to finish the review this week