control: ability to roundtrip partial control files

A concrete instance:
`prms_warmup` option has default of zero (in 5.2.1) but the default in pyPRMS is set 1. I have a control file without `prms_warmup`, round tripping the file causes output differences in the runs for runs longer than one year.

It sounds like updating the default in pyPRMS to the value in 5.2.1+ is good.

I see why populating all the control variables is desirable but it is also desirable to be able to roundtrip partial/incomplete control files. An option to do `complete` or maybe `roundtrip` mode on `.init()` could simply collect the keys and then delete these from the Control object before returning.