Make it possible to switch precision

[grey-nearing]

• Replace real var types in nrtype and throughout.
• Make new underscore variable type (e.g., _mrk).
• Make all quad precision the same user-defined (no need for a distinction between quad and double).

[bartnijssen]

Bumping this up in priority as this just came up again:

s/dp/mrk/g (almost) and then set mrk to desired precision

grep -i [a-z]dp[a-z] *

but then a little more carefully

[arbennett]

I worked on this a bit in my branch: https://github.com/arbennett/summa/tree/feature/single_precision
It compiles, but does not run due to internal tolerances not being met. The first one I ran into was the calculation of the wet bulb temperature. The second was in longwave balance. Lowering tolerances and recompiling can get around those, but after that I get a proper crash. From my experience it doesn't even run a single timestep.