This is done with a set of <date_valid_from>: <host_file.tif> pairs in
the run configuration under ['Epidemiology']['Host']['HostRasters'].
This is an optional argument. The 'HostRaster' argument in the
configuration is still needed, but is renamed as 'TargetRaster' to
avoid confusion.

Cocnurrent changes were made in epi model.

This may one day be replaced by some 'host preprocessing' type
of EWS component.

Aspects to improve:

*  Currently, the dynamic host map has to be handled both in
ews-epidmiology for run time, and here for post-run in order
to convert units. It might be neater to do once in one place only
 by moving the unit conversion functionality into the epi model.

 * Implement testing for handling of dynamic host data.