Improve hit counting in pyMCell

[adamhusar]

Currently, to count surface hits, one needs to load the whole file with surface hits as in example mcell_tests, branch pymcell_membranes:
examples_pymcell/0100_2_molecule_types_diffuse_in_box_w_plane/pymcell/0100_2_molecule_types_diffuse_in_box_w_plane.py

We will need a better way how to provide the hit trigger information to the python layer.