Add a geothermal heat flux

[noahdsmith]

There is a numerical correction added to the bedrock code to avoid rounding errors.

This issue also sets the heat flux at the base of the bedrock column to a constant value (rather than zero as at present).

hcondeep needs to be set to 3.0 as default (changed in an upgrade macro). it is currently 8.6 which is wrong.

hflux_geo is added as a parameter in the jules_soil namelist and also needs to be added via an upgrade macro. I think it also needs to be set to rmdi instead of 0.0 in jules_soil_mod.

Im not sure if bedrock is on in in an rose-stemt ests but this change will break them if it is.

https://github.com/noahdsmith/jules/tree/76_geothermal_heat_flux

[noahdsmith]

Code added, but not yet tested.

[eleanorgb]

here are the results of the bedrock simulation within jules. Look really good. Switching the geothermal heat flux off reduces the increase in temperature with depth to approx 1/10th of the observed value from the boreholes. So we need the geothermal heat flux term as well.