Add O2sol functions

[DocOtak]

With #23 merged, there appears to be a clearer path for adding missing functions. As such, here is my first attempt to add some functions, specifically two of the O2 gas solubility functions. I've also added the check functions and data.

These functions were directly ported from the matlab version, including comments/documentation. The functions were placed in the c source alphabetically by function name.

[efiring]

Thank you, I think this is almost ready. Please use 4-space indentation in the C function bodies, and remove the trailing whitespace.

[DocOtak]

Thanks @efiring, I've indented and think I squashed all the trailing whitespace. Shall I attempt a start at a CONTRIBUTING file for future reference?

[efiring]

@DocOtak see #27.