allow to specify default versions of selected software packages

[boegel]

We should allow to specify what the default version of selected software packages should be.
EasyBuild should then adjust the .modulerc file so that the generated module becomes the default version being loaded if no version is specified.

This is interesting for Python for example, where you'd want the latest 2.x release to be the default instead of any 3.x version.

Specifying this could be done in the EasyBuild config file, or in another file indicated in the EasyBuild config file, e.g., default_versions=/path/to/default_versions.csv.

[fgeorgatos]

I think this need could well be combined with the spin-off work, after #173 gets completed

To provide an answer to the why of this, think of the use-case:

• openfoam/2.1/gnu (default)
• openfoam/2.1/intel

ie. module load openfoam/2.1 should then bring up the gnu version regardless of sort order...
(I would put an asterisk in the namespace mapping file to denote defaults or, something like that)