RIKENFE - add variables to PLC IOC for monitoring separator voltages

# Feature Description

As a power supply technician, I would like to be able to see a log of voltage values for the separator plates on the RIKEN front end.

### Additional Information

- Currently, many values are read via MODBUS from the main Schneider PLC on RIKEN.
- This work is to add 6 more variables and addresses into the DB template files.
- Substitution files [here](https://github.com/ISISComputingGroup/EPICS-ioc/tree/master/SCHNDR/SCHNDR-IOC-01App/Db)
- Modifications potentially required in the [IOC startup file](https://github.com/ISISComputingGroup/EPICS-ioc/blob/master/SCHNDR/iocBoot/iocSCHNDR-IOC-01/devices/RIKENFE.cmd)
- As of writing, no _requirement_ for OPI changes, although would be helpful.

#### MODBUS variables and corresponding addresses:

MODBUS variable | Address | Unit
------------------- | -------- | -----
SEP1_NEG_HMI | %MW3004 | kV
SEP1_POS_HMI | %MW3002 | kV
SEP2_NEG_HMI | %MW3008 | kV
SEP2_POS_HMI | %MW3006 | kV
SEP3_NEG_HMI | %MW3012 | kV
SEP3_POS_HMI | %MW3010 | kV

_{See our [Git Workflow](https://isiscomputinggroup.github.io/ibex_developers_manual/processes/git_and_github/Git-workflow.html).}

# Acceptance Criteria
- Above values are logged
- Technicians are shown the changes (if on OPI) and where the log files are

# How to Review
- Check DB files are built and load successfully
- Files pass checks
- OPI changes adhere to agreed scheme (pass checks)

_{**To the reviewer: Make sure to [update submodules](https://isiscomputinggroup.github.io/ibex_developers_manual/processes/git_and_github/Git-workflow.html)!**}