* Added various views and helper functions used by the mining
computation but which may be re-used in other parts.
* Added mining computation update type and associated update function
* New constants: game.resources.weightBase (the value used to compute
weights from mining settings) and game.resources.extraction (the
quantity extracted in a day from a full provider at difficulty 0)
* 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.
