Feature/ncextract enhancement

[casadoj]

I have done some changes in the ncextract tool:

1. Now it produces one file (NetCDF or CSV) for each point of interest. Before, it used to produce one single file with all the points of interest together, which prevented following the development of the process, and caused loss of computing time when the tool stopped before finishing, so there was no output at all.
2. I added a filter based on the start and end of the study period. This solves partially issue Add filter by time in ncextract and catchstats #71

I have updated the test and the documentation in the Wiki.