New forecasts models : AROME PI & PIAF

Features:

• Allow user to specify the directory where to store the API result before un-gribbing it #28 by @ThomasBouche in #33
• Support for Météo-France models PIAF and AROME INSTANTANE #26 by @GratienDSX in #27

Fixes:

• Tests run in 2s instead of 20s by @yoyonel in #34
• Wrong function name to fetch bulletin #29 by @TimKeff in #31
• Wrong argument in a tutorial #30 by @TimKeff in #32