spectrum viewer: allow users to change flux units

Allow users to change flux units. Most freuqnely used ways to represent spectral fluxes  will be f_lambda, f_nu, and AB magnitude.  In cgs,

f_lambda [erg/s/cm2/A] 

f_nu [erg/s/cm2/Hz] 

These two are related to each other by 

lambda * f_lambda = nu * f_nu

(recall lambda * nu = c, and mind the unit difference; lambda is often Ansgtrom, while c is cm/s) 

Note that the conversion is thus wavelength/frequency dependent.

AB mag is given by mag = -2.5 * log10 (f_nu) - 48.60 

PFS uses nJy, which is 10^-23 erg/s/cm2/Hz.  So, a source with 1 nJy has an AB magnitude of 31.4.