[grids] minor fix in Triangle.quadrature
[grids] allow periodic boundaries for TriaGrid
[operators] fix L2ProductFunctionalP1 for order 1 in 1D
[operators] add lincomb method
[discretizations] allow to specify additional functionals
[operators] deprecate OperatorInterface.lincomb
[operators] move LincombOperator to pymor.operators.constructions
code cleanup
always use fully qualified imports
[tests] fix discretization fixtures

We now have TriaGrid as default, so diameters need to be adapted.