alternate multithreading map rebinning scheme

Change the way smf_rebinmap1.c splits the jobs between threads.

Old version:
* Map is divided into nw chunks. 
* each thread loops over full data set and skips samples that don't project into appropriate map range

New version:
* data are divided into nw chunks (chunking is by bolo)
* each thread has its own set of maps to accumulate into
* when accumulation is done, combine submaps (this step is multithreaded by map chunk)

Regression tests show that the maps change by roughly one part in 10^6 (in terms of map SNR). The rebinning routine runs roughly 4 times faster with this new scheme, and reduces the overall makemap runtime by about 10%.