Change numpy array calculation

from: http://tully.ups-tlse.fr/grizonnet/let-it-snow/issues/11

Numpy calculations need to be changed or maerged into a single file (utils.py ?) Here is the list where numpy is used:

• compute_HSmin Use numpy boolean filter and min to find the minimum elevation where there is snow.
• compute_HSmax Use numpy boolean filter, min and max to find the elevation where, from this value and above, there is only snow.
• compute_cloudpercent/snowpercent (also used in format_output.py)
• compute_cloud
• compute_stats_internal Use combinations of numpy array adn sum to make stats from image
• format_percent Format stats as percentages as in the publication