Tidy up the use of `raw_value`

[kodonnell]

As per #22 there's a bit of potential confusion around the semantics raw_value. That is, it could mean:

• (My original intention) - the 'raw' value after doing SunSpec conversion of bytes to a value (e.g. a Uint16) i.e. after calling from_registers.
• (The interpretation from Clearer outputs for different field types in the CLI and in a JSON export #22) - the 'raw' integer value of an enum.

That's my bad. Realistically, it's only used for scaling decimal fields, so I think the answer here is to refactor the code so that a 'normal' FieldValue doesn't have all this scaling stuff. Maybe create a ScaledFieldValue? Need to think about that. Then ensure there's only one value thing per (Scaled)FieldValue (not two as we currently have value and raw_value).