Parallel builds merged

The MPP and MPPX builds of Molpro have been merged and the decision to run in MPP or MPPX mode made a run-time option. To build parallel Molpro the -mpp flag to configure should be used; the -mppx flag is no longer required or valid. When running parallel Molpro, the --mpp option (default) can be used to specify MPP mode, and the --mppx option for MPPX mode. 2018-12-18