PR: Major improvements of the tool to evaluate groundwater recharge.

[jnsebgosselin]

Fixes #80 and Fixes #72

The goal of this PR is to improve the tool to evaluate groundwater recharge. This was included in WHAT in version 4.2.0-beta with some major ninja hacking. This PR cleans this up and includes some major performance improvements with the use of the cython package to improve some routines that were taking too much calculation time in Python.

In summary, this PR:

• Improves the ground-water recharge calculation time by 2 order of magnitude.
• Moved all things related to the assessment of groundwater recharge to a new module.
• Drop the use of the pyplot interface for plotting the results.
• Moved the calculation in its own thread so it doesn't freeze the GUI.
• Added a calculation progressbar.

[jnsebgosselin]

The plotting of the results is still a little bit slow, but this is going to be for another PR. This one is big enough.

The axis labels are not displayed correctly though, I need to fix that because I can merge.

[jnsebgosselin]

Ok, that was not the plotting that was slow. I forgot to remove a line of codes that I added for testing and that was saving the results to a numpy binary. This is what was causing the lag in the production of the figures.

[jnsebgosselin]

Here what features are still missing after this patch:

• Add the option to select the language for the graphs.
• Add the option to save the results in a csv or xls format
• Add the option to save the results in a npy format.
• Add the option to select the range of years over which recharge is assessed.
• Further improve the calculation time by multiprocessing.