Diffusion: tests, fixed coefficients in the solver, & tutorial

[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

[thorstenhater]

@jlubo I finished testing on our GPU cluster and everything looks fine now. If you could help to repeat this on your cluster and clarify the scaling we can merge this and release 0.11 including the fixes.