Add method to ED1d for calculating membrane resistance as a function of ion concentration

[lbibl]

Fixes/Resolves:

(replace this with the issue # fixed or resolved, if no issue exists then a brief statement of what this PR does)

Summary/Motivation:

Changes proposed in this PR:

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[johnson12742]

Which references talks about the membrane resistance as a function of ion concentration? I quickly glanced through a few and didn't find one that talks about this function.

I was recommended the Galama paper for it (https://doi.org/10.1016/j.memsci.2014.05.046)
Also, this is an empirical relationship (is that correct?). Maybe make a note of that.

Everything else looks good.

[hunterbarber]

@lbibl Can you clarify on the meaning of the spacer_conductivity_coefficient? I'm a little confused while going through the review.

[lbibl]

@lbibl Can you clarify on the meaning of the spacer_conductivity_coefficient? I'm a little confused while going through the review.

Added notes in doc: "We used a coefficient multiplied by the solution conductance, denoted by :math:\sigma, to account for the spacer's conductance shadowing effect." There are cases where the shadowing effect is treated as a coefficient to resistance rather than conductance. This would be a clearer naming with the added notation.

[lbibl]

Which references talks about the membrane resistance as a function of ion concentration? I quickly glanced through a few and didn't find one that talks about this function.

I was recommended the Galama paper for it (https://doi.org/10.1016/j.memsci.2014.05.046) Also, this is an empirical relationship (is that correct?). Maybe make a note of that.

Everything else looks good.

Updates on references underway.