Update get_data to support fully calibrated data

[jajmitchell]

A list of tasks necessary to extend

stixpy/stixpy/product/sources/science.py

Line 856 in fb3004c

def get_data(

to return fully calibrated data:

• livetime
• elut

and support returning different types of data:

• c = count
• cr = count rate
• dc = differential count
• dcr = differential count rate
• dcrf = differential count rate flux

obviously there are other combinations but not sure how useful they are can always add later as just a extra norm in the last step.

Potential function signature

get_data(<existing>, type='dcrf', livetime_correction=True, elut_correction=True)

In all subtasks need to handle cases where things change especially/only when summing over time:

• What area should be use when the pixel mask/rcr state change in a summed time interval - potentially best to use masked array and mask the interval
• What rcr should be returned (I don't think we return at the moment but prob should), max, a list or mask like area?

For a first cut could just raise an error like in the current visibly code and try and add later.

Moreover, a naming convention is defined for different rate-like values that may exist in > the instrument class, and throughout the package, as

"something Rate" refers to something s^-1
"Differential something Rate" refers to something s^-1 keV^-1
"Differential something Flux" refers to something s^-1 keV^-1 cm^-2
(E.g., photon rate is photons s^-1, differential photon rate is photons s^-1 keV^-1, and >differential photon flux is photons s^-1 keV^-1 cm^-2.
stolen from here sunpy/sunkit-spex#126