FieldIO instrumentation simplifications

[pixelzoom]

PDL was used as an exemplar for instrumenting Gas Properties. While working on phetsims/gas-properties#77 and phetsims/gas-properties#231 with @zepumph, we noted the following things about the implementation of FieldIO:

• toStateObject is unnecessary. The default implementation that is derived from stateSchema does the same thing.
• defaultDeserializationMethod is unnecessary. (Ask @zepumph why.)
• applyState would be unnecessary if stateSchema used ReferenceArrayIO instead of ArrayIO. See ReferenceArrayIO's default applyState (line 37).

According to @zepumph, the above changes will change the PhET-iO API, but will not require migration processors.

[samreid]

I applied the changes above and tested in the state wrapper (I tested a VSM screen and the sampling screen). Seems to be working OK. @matthew-blackman can you please review/test?