## Partial numerical differentiation

It would be nice to have the possibility in Dumux to choose which equation is differentiated with respect to which variable. For example if we solve a richardsni problem where we assume that the solution of richards equation is independent of the temperature (e.g. if densities and viscosities are constant) we do not need to calculate the derivatives of richards equation with respect to temperature.

This could be realized by introducing some table that lists if `eqIdx`

should be differentiated
with respect to `pvIdx`

. Furthermore, this could also help to decouple problems or to implement other solvers than Newton's method.

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