Add Design.field attribute

[albireox]

Restores the Design.field reference using a property with a custom query instead of a Peewee ForeignKeyField.

The logic is:

• If $RS_VERSION is not defined, fails.
• If there are multiple fields associated with a design for the same $RS_VERSION, fails.
• If there is only one field associated with a design for the RS version, return that one.
• If there are no fields associated with a design for the current RS version, removes the RS field condition and removes the first field (should apply only to commissioning designs without an associated RS run).

[johndonor3]

Looks good to me!