Made tests independent of astropy.units version (which removes parentheses in Astropy 0.3 if denominator only has a single unit)
Re-implemented flux properties and only warn when necessary about using default meta-data values.
Tidied up analysis: warnings for meta-data are only raised when the values are needed and the defaults are used, exceptions for strict meta-data are only raised when needed, Metadata class(es) now implement type checking, and more of the analysis properties are returned as quantities.