Add Leith+E (#251)

* Add Leith+E

This commit adds the 2D Leith+E closure, which uses a modified 2D Leith biharmonic viscosity paired with a harmonic backscatter. ('Modified' here is not used in the same sense as 'modified 2D Leith'; it just means that the biharmonic coefficient is modified to account for enstrophy backscatter.) Variables are often named 'leithy' to refer to Leith+E.

The parameterization is controlled by three main entries in user_nl_mom:
1. USE_LEITHY = True
2. LEITH_CK = 1.0
3. LEITH_BI_CONST = 8.0

To use Leith+E you should have LAPLACIAN=True and BIHARMONIC=True. (It doesn't hurt to be explicit and also set LEITH_AH=False, along with any other viscous closures, but this is not required. If USE_LEITHY=True it will not use any of the other schemes. It does use the background value of the biharmonic coefficient as a minimum, but ignores the
background harmonic value.) LEITH_CK is the fraction of energy dissipated by the biharmonic term that gets backscattered by the harmonic term (it's a target; the backscatter rate is not exact.) Recommended values between 0 and 1. LEITH_BI_CONST is Upsilon^6 where Upsilon is the ratio between the grid scale and the dissipation scale for enstrophy.
Values should be greater than or equal to 1; 8 is a good place to start.

The code is sensitive to the background value of Ah; specifically, if Ah is too large, the code is unstable. This is because the backscatter coefficient is proportional to Ah, and if Ah is large then you get large backscatter. If your code is unstable, consider reducing, e.g., `AH_VEL_SCALE`.

* Background Ah

This commit updates the code so that it uses the background Ah as
a minimum. Previously, if `SMAGORINSKY_AH = True`, Leith+E would
use the Smag value of Ah as the minimum, which is incorrect.

* Improve logging

Removed `do_not_log` condition on `USE_LEITHY`

* Fix Leithy Logic

Added one line to fix the fact that the code would only work as
intended if either (i) writing out Ah_h, or (ii) in debug mode.
Also swapped .le. and .lt. for <= and <.