fix documentation
[parameters.base] bugfix
[functions.basic] allow zero-dimensional domains in GenericFunction
[grids.rect] bugfix
[la.interfaces] bugfix
[grids.referenceelements] fix caching for ReferenceElements
[gui.glumpy] add suppport for RectGrid and face-DOFS
[operators.basic] add ConstantOperator
[operators.fv] add nonlinear fv advection operator
[algorithms.timestepping] add explicit Euler scheme
[tools.inplace] add fast indexed inplace operations