Unfinished evaporation stuff

Adding evaporation calculations. This is a first approximation to the
problem, soon to be substantially revised.
Refactor evaporation

Code has been refactored and this second approximation looks better.
Untested.
Tested and corrected evaporation
Evaporation now works with numpy arrays
Add documentation for gerarda

gerarda is the command line program that will be calculating spatial
evaporation. Documentation only; program not written yet.
Simplify CliApp by abolishing check_configuration

CliApp separated read_configuration from check_configuration, but this
was suboptimal and confusing. Although now it interally keeps them
separate (in fact checking is part of reading), it no longer encourages
overriding of check_configuration in subclasses, instead specifying that
the configuration should be checked in overrides of read_configuration.
Add gerarda
Minor fix in fordonia testing code
Remove timeseries.datetime_from_iso; use iso8601