Allow ProblemStat to be constructed from pre-basis factory

Merged Praetorius, Simon requested to merge feature/problemstat_construction into master


Instead of first creating a global basis and then passing this together with a grid to a ProblemStat, the latter can be constructed directly from the underlying pre-basis factory. This allows to write code like

Grid grid = ...;
ProblemStat prob{"name", grid, lagrange<1>()};
Edited by Praetorius, Simon

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