pwkit/lsqmdl.py: utility to 'freeze' model function with given parameters
pwkit/lsqmdl.py: don't let rchisq values linger from run to run
pwkit/lsqmdl.py: remove extraneous 'npar' argument to Model.set_func()
pwkit/lsqmdl.py: shorter, more consistent naming for param attrs

'paramnames' becomes 'pnames' and 'perror' becomes 'puncerts'.