Remove pointers in multiply module

[tkoskela]

This PR removes pointers to part_array in multiply_module and passes slices of part_array to subroutines instead. It also tries to clarify the indexing of part_array to make it clearer which slice is being passed.

I'm open to opinions on whether or not this is a good idea. It's more of a style change than anything else, although it should make the code more robust against the kind of memory errors in #280. I would argue this is a more Fortran style use of arrays, compared to the C-style use of pointers. It does make the subroutine calls somewhat longer