Add a geothermal heat flux

[noahdsmith]

(#152)

l_bedrock is not currently available in the um. However, the meta-deta needs to be replicated and an upgrade macro created.

There is a bug fix:
The bedrock wasnt working properly because of rounding erros and small changes in temperature - a numerical correction is added to the bedrock code to avoid these 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. It is set to rmdi

If bedrock is on in a rose-stem tests this change will break it as it changes the calculation of the deep soil temperature (before the change to hflux_geo)

[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.