Merge all skill notebooks into one

[casadoj]

Currently, there are 5 notebooks with skill computations:

• Seasonal: it was only a test
• NWP with all lead times
• NWP at lead time ranges (daily or specific)
• COMB at all lead times
• COMB at lead time ranges (daily or specific)

Tasks:

• I think that the seasonal notebook can be removed
• I must find a way to combine the NWP and COMB notebooks at lead time ranges
• I think that the notebooks for all lead times (both NWP and COMB) can be removed. The analysis at all lead times could be done specifying two lead time ranges [0, 48] and a minimum lead time of 60. In this way, I can also remove 2 of the configuration files.

[casadoj]

I have created the notebook 6.0.2_skill_leadtime.ipynb that combines the NWP and COMB experiments for the lead time ranges and the daily analysis.

For the daily analysis, look at issue #22

[casadoj]

I have merged the two notebooks computing hits for NWP and COMB into a single notebook.