Make it possible for other python code to use Parser directly

[yyyliu]

Besides the command line interface, we should make it possible for other people to use the Parser class for compiling a multiverse from within python.

Now it's possible to import and use the Parser class if they import the boba package, however, Parser prints and exits upon any error, which is undesirable.

The first step is to modify Parser to throw an exception instead of print and exit. We might think if other aspects might need improvement.