Add ability to configure Header section

[tpgillam]

Hello! We (also @db434) would like the ability to change the Header section of the SIF file that is generated (e.g. mesh & results locations).

This PR:

• Adds a header dictionary to the Simulation class which stores parameters
• Slightly expands the tests, such that the files stored in git are now used as a reference, and we check for exact matching against them. (This seemed the simplest way to check for unexpected regressions more generally)
• Adds a sif_filename property to the Simulation class, so that we can write to a different filename when needed.

Would appreciate your thoughts & feedback, thanks!

[arvedes]

Hi, that looks very good, thanks for the contribution!

I saw that you changed some of the filepaths to be constructed using os.path.join, we should do that consistently. I just opened a new issue for that.

[tpgillam]

That's great, thanks for the very fast turnaround!