Saving pickle fails if git is not available

[ivvv]

In the io.py module, when saving the posteriors and priors the code needs git_version() which calls git. If the command fails then an exception is raised.

This is inconvenient when e.g. sending an XID+ run on a grid, where the grid nodes do not necessarily have git installed. We tested and adjusted the XID+ script on a desktop where it was available, so there was no warning or anything. And as a result, when we run the actual processing on a grid, it failed with an exception after 50 hours. Not very helpful.

Ideally git_version() should not raise an exception but simply assign an unknown version (or something similar) with a warning of a possible mismatch.