Add diagnostics + minor refactoring

Adds a new "Diagnostics" module which provides ready-bake implementations of a few common permafrost metrics:

• Zero annual amplitude
• Thaw depth
• Active layer thickness
• Permafrost table depth
• Permafrost base depth
• Mean annual ground temperature
• Generic spatial integration for extensive variables (e.g. energy)

In addition, the Interface module is removed (this is confusing, in hindsight, since "interface" has a specific meaning in permafrost, i.e. it is synonymous with freeze front (?)) and the corresponding types/functions are now included at the top level in the CryoGrid module. All submodules now explicitly import these types.

CryoGridSetup also now requires the given Grid to be "unitful", i.e. grid values must have length units. These unit types are still discarded internally when CRYOGRID_DEBUG is disabled.