Fix for the pcs.read user function.
The problem was caught by the new Pcs.test_pcs_copy_different_spins system test. If the spin system does not exist in the current data pipe, but data for it is present in the PCS file, the pcs.read user function would terminate in a TypeError.
Authored by: bugman 2015-03-03
Parent: [r27741]
Child: [r27743]
