Input & output units, documentation, etc.

[jetuk]

pycatchmod is a bit funny as it takes inputs in mm/day for rainfall and PET, but also wants an area in km^2. The output flow is then Ml/day. None of these are SI units and the output is different to the input.

1. This needs better documentation in the docstrings, etc.
2. There is an argument to make the whole library work with SI units (perhaps with the exception of days for time). So input would b m/day output would be m^3/day. However this would be a backward incompatible change that could be quite dangerous.