User selectable phase function normalization

One of the pain points of working with Mie scattering functions is figuring out exactly how the phase function is normalized. miepython defaults to normalizing the integral of the phase function over all directions to equal the albedo. This version now allows one to normalize to a range of common numbers:

• albedo : single scattering albedo
• one : unity
• 4pi : since there are 4π steradians in a sphere
• qsca : the scattering efficiency
• qext : the extinction efficiency
• bohren : 4π x² Qsca
• wiscombe : π x² Qsca

The normalization is done by specifying a different normalization

    intensity = miepython.i_unpolarized(m, x, mu, norm='wiscombe')

See the normalization docs on https://miepython.readthedocs.io/en/latest/03a_normalization.html for details