Investigate archiving/decimating strings fields: policies

As of right now if you specify a set of fields to be archived that contains both string fields and numerical fields and ask it to decimate, this will cause errors as it cannot calculate the mean of a string for example. It would make sense to use a different policy to specify how to store the string PV fields, but you can only use one policy per PV as I understand. A possible solution to this could be to define a new storage medium for string fields.

To see error first hand, uncomment line 117 `#RDBL` in `policies.py` and rebuild/run.

Start in policies.py at line 117 by uncommenting #RDBL, then rebuild and run the configuration to reproduce the mixed string/numeric decimation error. Trace how one policy is applied to both field types and determine a policy or storage-medium outcome that handles string PV fields without breaking numerical decimation.

Written by the indexing model from the issue text.