Store SCM reference on a given processing

[rafamanzo]

This allows, if necessary, to check back removed repositories in order to correlate code with results in the future.

[danielkza]

We also should try to infer those references for the most recent processings of existing repositories if we can.

[rafamanzo]

Every SCM (source code manager) attributes to the current version being viewed and all the stored ones a revision number. Git for example uses a hash.

We want to store that reference within each processing as a string. This will enable us to proceed with #204 saving disk space.