numpy version issue between fitsio and pymc

[combet]

The (older) numpy version allowing for pymc to run is not compatible with what fitsio requires. As a consequence clusters_data.py crashes... Solutions to be explored:

1. Quick fix: create 2 python environments, one compatible from clusters_data to clusters_getbackground and one compatible from clusters_mass. We would then turn on/off the environments as needed.

2. Longer term solution: migrate to PyMC3 instead of PyMC. The latter does not seem to be maintained anymore and moving to PyMC3 should allow for more recent numpy versions. A specific pymc3 branch has been created for this purpose.