This commit adds a thickness calculation using an integral form of the
hypsometric equation with virtual temperature adjustment. This assumes
hydrostatic balance. Also included are two tests of this calculation,
one using matching data from other tests and one using a simple dry
isothermal layer.

Based on feedback, corrects the thickness formula to use d(ln(pres)) instead
of d(pres)/pres. While these are equivalent in the continuous case, they do
not give the same results in the discrete case here. Tests have likewise
been updated to account for this change in formula (which should hopefully
also resolve the legacy test failure).

Based on review feedback, updates the optional mixing ratio to be handled conditionally rather than with an array of zeros (if none is given), as well as conditionally calling get_layer only when a bottom/depth is specified. Adds two additional tests to cover all four scenarios (full layer-moist, subset-moist, full layer-dry, subset-dry). Also adds a citation for the thickness formula.

The recent addition of the static energy calculations unfortunately caused a messy rebase conflict where the new functions got interleaved. I thought I got it all cleaned up, but unfortunately missed this exporter statement.