Fix bug in C implementation of HEALPix map smoothing

Particles should be accumulated to a single pixel if their angular smoothing length times kernel gamma is less than the pixel radius. The gamma factor had been left out so the code stopped smoothing at a scale ~1.9 times larger than necessary. It was probably also communicating more particles than were accumulated to the map because the gamma factor was included when deciding which MPI ranks particles should be sent to.

Marked as draft for now because I'd like to do some more testing.