Soft bounds for 750M
lockin ADC return in V
prologix factory functions

These create a module-level registry of prologix controllers, so if one
is instantiated for an address which already exists, the existing one is
returned instead of making a new one. This fixes a bug related to
caching of the _addr variable.

There's still an issue of the equivalence of certain addresses (e.g.,
equivalence of Serial(0) and Serial('COM1'), etc. Fix that later
fixed bug in density plot
pass density plot kwargs to imshow
added averager decorator to util
removed ser_avail variable
Modify plotgen to not vomit a traceback on keyboard interrupt