Fixed cache error in ogr_layer_from_stations

ogr_layer_from_stations was caching the location of stations in a file
named timeseries_{id}_point. This might not be unique if caching stuff
from more than one Enhydris installations. The filename now includes the
base_url.
interpolate_spatially writes to a given band

interpolate_spatially was creating a new band in the dataset which
contained the mask; this has now changed; its caller must create the new
band in any raster it likes and pass it as an argument.
spatial.py API documentation moved to docs
Added h_integrate (untested); and some other changes in spatial

Changed the term "interpolation" to "integration"; also fixed a bug
where the time series cache files might not have unique filenames.