[core] fix sid generation in case __init__ has no default arguments
[tests] remove obsolete test
[tests] add another pickling test for functions
[tests] fix test for gmsh
[tests] test if dumps_function is only used when necessary
[analyticalproblems] pickle Burgers problems without dumping the flux functions
[tests] test if grids and vector arrays can be pickled without dumps_function
[analyticalproblems] make data functions picklable
[la] let induced_norm return a picklable class
[tools] make FrozenDict picklable
[discretizations] make DiscretizationBase picklable
[tests] add pickling tests for discretizations
[tests] handle instance methods in assert_is_equal
[operators] do not pickle last assembled operator when pickling NumpyMatrixBasedOperator