resample cellarea continue

[lazarusA]

getting this #800 #801 and #730 in.

This is getting too long. So, I will merge the current changes, and then we can refine later if needed. Note the following:

• In the Manual section I added all methods and data sources. Currently, the only method pointing to a new section is cellarea since is the only one that has additional documentation. The remaining ones, point out to the API docs. Hence, if we need more docs on the different methods, please let's first add the API docs at the top of each section, before going into more details.
• links to methods (there are not that nice), hence populating with extended docs would be good (we could shorten them then- being also maybe more stable in the long run).
• this PR contains a tutorial on resample and reprojection with resample and ArchGDAL internals for the transformations.
• another reorganization of topics, trying to be consistent with things that should go into the manual (base functions) and tutorials (a combinations of them to create workflows)

[asinghvi17]

Thanks for the port! Added a few suggestions.