Enhancements for reading .nc: subselection and multiple variables

[Datseris]

1: Allow the function to take as input a tuple of indices, e.g. (:, :, 1:5)
and only load this part, and correctly and instantly make it a ClimArray, which
can solve "large memory" or "large data" problems. This funcionality
must be sure to load the correct ranges of dimensions as well though!

2: Allow reading multiple variables at once. This has the performance benefit of not re-creating dimensions all the time.

[Datseris]

Part 1 has been done in 0.14. Part 2 still pending! The remaining part 2 is a good first issue!