add LognormaModel()
add expr to set_paramval(), fix typo
use set_paramval more uniformly
refactor: Model.set_paramval -> Model.set_param
set most sigma params to have min=0
move pngmath/mathjax option to imported extensions.py, copied in Makefile
move pngmath/mathjax option to imported extensions.py, copied in Makefile
tweaked intial values for peak functions
better definition for Pearson-VII function
using np.polval for Polynomial model
ensure that all components of a model share full set of parameters