Units as calcs

[mikofski]

• use inspect.argspec or provide args order in formulas param files
• wrap function with ureg.wraps(ret units, arg units)(f) in formulas, remove from formulas py file
• update formulas registry with args and units, also formulas subclass and formula layer, use _meta_names
• fix outputs PLURAL not singular
• fixes for pvlib, lots of missing outputs, fix fields that should pandas, and use values to return ndarray
• fix if return value scalar then return unit in wrapper must be scalar too!
• YAY! it works!