Modularise megphys script, making it cleaner and more easily modifiable

[drewsberry]

Pretty self-explanatory. Makes functions for each corresponding option, to make it more easily readable and modifiable. I haven't done that for test, because it's a shell built-in function.

[hmac]

Looks good. While you're at it, could you update the usage with test and update_tests?

[drewsberry]

Well that was a unsuccessful commit message. It was supposed to say:

Update usage with test and update_tests

[drewsberry]

Amended