Fix coefficients in the diffusion solver

[thorstenhater]

By careful re-analysis of the diffusion solver we found that the coeffiecients of the LHS need an additional factor of the CV Volume.

Scopes be creeping. During the analysis it became apparent that the during coupling to the cable equation proper raises at lot of potential problems.

Conversion of ionic current densities $i_X$ to mass transfer is $\dot c_X = \frac{i_XA}{q F V}$ with volume, area, Faraday's constant, and charge. Now, people are interested in using neutral species $n$, ie $q=0$ for which we also should expect $i_n=0$. Yet we need to special case on this (and assert zero current) to avoid ill-defined division. That's the smaller problem.

The more worrying issue is this:
We construct a coupling term from the cable equation to the ionic diffusion via $\dot c_X = \frac{i_XA}{q F V}$. Yet, there's no backreaction to the cable equation, unless the user explicitly constructs it. How? Well, the way ions should feed back to the voltage is this:
iX = g(U-eX)
this is the conductance model and should be found in an NMODL file and
eX $\sim \ln\frac{X_i}{X_o}$
which is the Nernst equation found, again, in a builtin NMODL file. But, for technical reasons we had to invent a special diffusive concentration Xd which is not identical to Xi. For proper behaviour, we should have used Xd instead of Xi in the Nernst term above. So, a non-standard Nernst module needs to be used.

This brings me to the great change in this context: remove the offending term and carefully document how to retrofit it in NMODL and add the proper Nernst model, too.

fixes #2145
requires #2209

[jlubo]

The functionality seems fine, at least on CPU single-core systems! I've checked this by running several tests. First of all, and most importantly, all tests from #2209 pass, including the previously commented ones (I guess @thorstenhater you checked this as well). Also the tests of my single- and multi-compartment network simulations (which are for a more particular model, but relatively extensive) all pass. The code too looks fine to me, too, as far as I can judge. Regarding GPU systems, I guess #2244 will be needed as well!? What else will be needed here?

It's also very nice to have the tutorial! However, I would suggest to split this into a documentation and a tutorial page. I have a draft for a documentation page on the diffusion mechanism that I started to write a while ago - maybe we could create another PR for that and put parts of the tutorial here into that one.

[jlubo]

This should not be 0 in the first place... Would be good to ensure this upstream and otherwise throw a warning.