MAPS: FZJ digital fermi chopper IOC - allow setting of frequency via OPI and directly via PV

Due to the data format of the fields in the MBBO record for the frequency setpoint, it's not possible to set the PV directly via `caget`, for example, whereas it is via the OPI.

Currently a "fix" has been added to the MAPS `set_ei` script to allow this. However, it would be preferable to move this logic to the IOC to allow both methods to set the PV.

The MERLIN Fermi chopper IOC has a suitable fix which could be used in this case: It has a separate record for the OPI which reflects the standard one.