Drop factor of 0.83 in contribution function

[wtbarnes]

The contribution function method currently includes a factor of $n_H/n_e\approx0.83$. This should be dropped as users may want to make different assumptions when it comes to what this ratio is. This means that the resulting EM is $n_Hn_e$ rather than $n_e^2$.

[wtbarnes]

The AIA response function example will need to be adjusted appropriately if this change as made as well as the documentation for the contribution function method to make sure that it is clear to users that this factor is not included.

[wtbarnes]

Alternatively, we could use proton_to_electron_ratio. The most important thing is that we are consistent anywhere this factor comes up. The three options then are:

1. Keep 0.83
2. Use the proton-electron ratio
3. Drop this factor entirely such that $EM\sim n_Hn_e$ rather than $n_e^2$

[wtbarnes]

Closed by #273. The decision is to drop the factor entirely (option 3) in the contribution function, use an assumed ratio to compute $n_H$ for the emissivity, and then interpret the EM in intensity as $n_en_H$.