added valid_commands to builtins.py for initial valid commands (no parers needed)
added .pre_slope, .pre_offset, and .norm_coefs output from pre_edge
added support for plugins to define initializeLarchPlugin function that can be used to initialize a plugin: symbol table is passed in
use  initializeLarchPlugin to add plot commands as valid_commands
altered initializeLarchPlugin to take _larch argument, to more closely match plugins and give more full access
using new initializeLarchPlugin method
better saving of pre_edge parameters: Thanks BR for the patch\!
better example/doc of pre_edge_kws
turn off auto-h5 conversion for GSE escan data
added (forked) version of uncertainties package
using improved, included uncertainties package to calculate uncertainties on arbitrary constraint functions
altered imports for fitting submodule