Distinguish between counts and count_rate in codebase

[hayesla]

at the moment throughout the codebase the term counts is used for both counts and count rate (i.e. counts/s/keV). just for simplicity and for future development this should be clarified in codebase and docs

[samaloney]

Borrowing from sunkit-spex sunpy/sunkit-spex#126 (comment)

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

So initially I'd suggest to return a dictionary with the relevant name count rate differential count rate, and differential count flux or something or possibly the set some attribute which controls what is returned.