Properly fix when OutputFieldsDefault writes output

[JamesMcClung]

• Extract/factor out common behavior of pfield params and tfield params
• Fix behavior of when to do a pfd/tfd write after checkpoint
• Fix behavior of when to accumulate for tfd
• Add comments and generally improve code readability
• Add tests for the above

[JamesMcClung]

I'd like to remove next_out as a parameter to do_out() and just calculate whether the given timestep is an "out" step. This would mean removing {p,t}field_next_ and ignoring the result of do_out() when restarting from a checkpoint. The trade is a (presumedly negligible) performance decrease for eliminating the surprising current behavior of do_out().

Although one unaesthetic thing would be having to do the math in both tfield.do_out() and tfield.do_accum(), so maybe it's not worth it.

[germasch]

Really nicely done. I hadn't expected you'd actually do the unit test stuff, but that's great.