gvar version 12.1

Adds batch capability to gvar.sample and gvar.raniter so samples can be created in large batches, which is much more efficient if large numbers of samples are needed. Also adds similar capability to gvar.BufferDict when making clones. These changes are useful for (fast) Monte Carlo simulations: see the section on Sampling GVars in the tutorial.

In addition to several minor bug fixes and optimizations (e.g., reduced import time), this version also updates the random number generator to conform with changes in numpy v1.17.