ClimaLand Diagnostics, update

[AlexisRenchon]

Purpose

This is a second major PR to introduce diagnostics in ClimaLand.

To do:

• [ ] union type ::BucketModel, ::SoilCanopyModel
• [ ] more tests
• [ ] better docs
• [ ] macro to reduce code duplication in compute_ and add_diag ( ? )
• [ ] fix names, units, comments of SoilCanopyModel diagnostics variables
• [ ] add more default methods, period of averaging, what variables, etc.

[AlexisRenchon]

@Sbozzolo just listing our to do list for now.