Implemented resource regeneration computation in the
verse.compute_provider_regeneration() function.
Created the PLANET_RES_REGEN update type, and added the corresponding
implementation.
Added resource regeneration constants registration.
* Added structures for resource definitions, natural resources
definitions, resource providers, empire resources and empire mining
settings (both empire-wide and planet-specific).
* Added a few common utility functions to the SQL test suite. These
functions allow test initialisation to be a little shorter.
* Added "MINE" production type and an associated building definition.
The production will not be added to the XML dump or to the output of the
planets summary page, as this is extremely temporary.
