Clean up unified convection scheme and general convection pre/post

[lisa-bengtsson]

Description

The starting point of the unified convection scheme is at large inherited from the GF scheme. In order to be transitioned to operations it needs to be optimized for speed, not re-define CCPP defined physical constants, remove old goto statements etc. Many comments have been provided by code managers in PR #56. In addition, there is a large overlap in pre/post for the different convection schemes that should be combined.

Solution

Address comments in PR #56, and potential additional optimizations. Combine pre/post for convection schemes that need them.