FIX: Samplemanager clobbering th1 with th2 dataset

Samplemanager runs LoadBedPostDataHelper() for each f,th,ph file it
finds.  This bug only appears if you have more than one of these files.
It would overwrite f1,th1,ph1 with f2,th2,phi2.  This has two issues:
 1. Most of the second dataset is very isotropic, causing the terrible
    tracking results observed.
 2. The second thread is left uninitialized, which further increases
    potential for chaos.
Axe samplemanager's from-scratch seed logic and replace with ptx2's.

This gives us:
 - nparticle support
 - seed mask support
Which constitute the primary use cases for this software.