Support depth in meters for mixed_layer_cape_cin

[dopplershift]

Currently, depth is usable in mixed_layer_cape_cin only if given as pressure. If you try to pass e.g. 500 * units.m, you get a dimensionality error, because it uses that to figure out what levels to strip out of the sounding. Currently, though, the docs say that any argument for mixed_layer should work.