Toniof 488 cubeconv

[TonioF]

Addresses #488 . A new document cubeconv.md replaces the old one cubespec.md.
A new module treatascube.py with methods split_cube and merge_cube has been implemented; these methods have been used to replace assert_cube.py in the instances where it has been used.
(On a side note, in most cases it was enough to use split_cube).

Remaining issues:

• split_cube internally calls normalize, but without normalizing it to use lat and lon. This does not happen because the dataset would thereby changed in a way that it cannot be safely converted back.
• For the method resample_in_time in temporal.py it is not enough to use split_cube and merge_cube, as temporarily removed variables might also carry a time dimension. However, this case will not occur often.
• For the method compute_dataset in compute.py, it is still necessary to implement that all datasets are first resampled to a common CRS.

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

Remember to close associated issues after merge!