Changes

I added an option to improve dry, convecting boundary layers.
The new option is triggered by setting l_vary_convect_depth=.true.
The option reduces the spurious surface spike in thlp2 in Wangara.

The option calculates the Deardorff convective velocity using
a variable estimate of layer depth based on the depth
over which wpthlp is positive near the ground.
thlp2_sfc is then diagnosed by assuming maximal correlation
between thl and vertical velocity, w.

Currently, l_vary_convect_depth is set to false,
and hence the answers remain BFB.

For #905.