Merge pull request #139 from ens-lgil/master

Update the variation tracks names replacing 'variants' by 'short variant...
Updated to match new assembly variables
Reinstated date-checking code, and added check for assembly name/version changes, in case of human error in populating the core databases
Added a line of documentation about debug mode
Changed hardcoded variation tracks to ones taken from species ini file
Added alternative default variation tracks for species that have a variation database (and, optionally, structural variants) but no "special" variation sources such as 1000 Genomes