add electronCount for single frame

[swelborn]

add individual frame counting

• no support for row-dark currently
• had to change ctor for vectorToPyArray, see here: https://github.com/pybind/pybind11/issues/1042#issuecomment-2262727578 . This may be a numpy 2 thing. Have to test this with normal pipeline when CI finishes.
• outputs will be a SparseArray with one frame/scan shape = (1, 1) and no metadata. This is to take advantage of methods in SparseArray