re-factor pfsConfig creation in fps

Re-factor pfsConfig creation in fps, that will follow those steps:

1. create the pfsConfig from the design
2. update pfsConfig.pfiNominal field, with newly computed x,y position, accounting for proper motion and airmass.
3. update pfsConfig.pfiCenter field, with the final convergence results
4. Write the pfsConfig to disk
5. Insert pfsConfig in opdb

The routines to calculate the new pfiNominal position are not implemented yet, but it should just be a call to pfs_utils basically, i'll leave a blank space for now.