Add partial support PROJ ellipsoid definitions

[Rennzie]

PROJ has rich support for defining an ellipsoid in a proj string. RG only supports the builtins or definitions via ellps=a,rf syntax. This PR adds support for reworking a and rf parameters in a proj string to ellps=a,rf. This support is added to parse_proj as it felt like the cleanest approach.

I opted for ignoring ALL other cases as they seemed more difficult to fix. I've added some documentation to parse_proj to indicate to users what they can expect.

I've also tacked on a small tidyup to proj strings whereby a scaling k parameter gets converted to k_0.

closes #61

[busstoptaktik]

Two nitpicking comments and two general hoorays :-)