A Fixer Upper (v0.2.1)

• Added viridis colormap #75
• Added NEXRAD level 2 example #45
• Split get_speed_dir into get_wind_speed and get_wind_dir #66
• Fixed clipping of wind barbs on skewT #54
• Fixed broken NIDS attributes on Python 3 #55
• Fixed bug with units on heat_index #56
• Moved to use logging rather than warning for issues beyond user control #51
• Clarified units in get_wind_components #46
• Other minor documentation fixes