AST-1562 Remove h5parm assignments

After removing SYSTEM when including schaapcommon on !1265 (merged), the compiler started warning about using the deprecated implicitly declared assignment operator of H5Parm objects. These objects are based on HDF5 objects which have a copy constructor but lack a copy-assignment operator. This MR avoids using the assignment operator for H5Parm objects and thereby removes the compiler warnings.

Another, better reason for this MR: schaapcommon!143 (merged) deletes the assignment operator from H5Parm, so the compiler now gives an error when SagecalPredict uses it.