Have bitia put timestamp in output filename

This reverts the most important change of 2cbbbf8.
gerarda: Use timestamp in filename
Fix PEP8 and other trivial issues
Fix error in usage of np.vectorize (fixes #10)
Minor reorganization of documentation files

I put user documentation in a different directory than developer
documentation. This makes it possible for a user application to have the
same name as a python module.
Rename pond to enhydris_cache
Rename fordonia to aggregate
Rename bitia to spatialize
Rename gerarda to vaporize
Fix filename: "spatilize" -> "spatialize"
Recalculate output when there is more data (fixes #7)
CLI applications in alphabetical order in docs

When the CLI applications were renamed, we had failed to reorder their
list in the documentation.
Document SoilWaterBalance class (not yet written)
Add pthelma.SoilWaterBalance class
Fix a python3 problem in SoilWaterBalance tests
Update CHANGES.txt
Fix CLI application rename remnants

pthelma.iss contained some obsolete references to "pond" and "fordonia",
which were changed to "enhydris_cache" and "aggregate".
Rename spatialize number_of_output_files parameter

The parameter was number_of_output_files in the documentation, but
number_of_files in the code. Changed to the more precise
number_of_output_files.
Fix vaporize not working with DEM

When a DEM instead of a constant value was being passed as the
"elevation" parameter in the configuration file of vaporize, it was
raising an exception. This was fixed.
Ignore grid points with nodata in vaporize

If a GeoTIFF file contained cells with nodata, as is usually the case,
vaporize wasn't ignoring them; instead, it was using the nodata value
(usually a number close to -4e38) in the calculations. Now it properly
converts nodata values to NaN upon reading the file. This was
particularly important for the case of elevation, where the nodata value
was failing the range check.