Add unit tests

[loeiten]

Although the code has been tested several times manually, automatic tests are needed in order to more easily test whether a commit breaks expected behaviour (and possibly detect undetected bugs).

See discussion at #19 (comment) for details.

[johnomotani]

Partially addressed by #38

[johnomotani]

Would be nice to have a set of tests for BoutOptionsFile that read/write a BOUT.inp, e.g. to test #74.