Merge 3D CG features

Notable features:

• DG to CG gather with min, max, sum, mean_area, mean_volume
• CG to DG array, allowing CG data
• CG exports
• Complete rewrite of mercator.py, removal of compute_csr, replaced compute_csr_cg
• fixed_time now works again, and slimdata are usable for 3D simulations
• Last un-transposed fields are now in the new shape

@dobbelaeret and @cscherpereel, I'm waiting for your approval to merge this into dev