Bumps

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* restore DREAM fitter efficiency (it should now require fewer burn-in steps)
* errplot.reload_errors allows full path to model file
* clip values within bounds at start of fit so constraints aren't infinite
* allow *--entropy=gmm|mvn|wnn|llf* to specify entropy estimation algorithm
* allow duplicate parameter names in model on reload
* expand tilde in path names
* GUI: restore parallel processing
* GUI: suppress uncertainty updates during fit to avoid memory leak
* disable broken fitters: particle swarm, random lines, snobfit
* minor doc changes

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* improved handling of parameters for to_dict() json pickling

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* parallel fitting suppressed in GUI for now---need to reuse thread pool
* support *limits=(min, max)* for pm and pmp parameter ranges
* cleaner handling of single/multiple fit specification
* fix *--entropy* command line option
* better support for pathlib with virtual file system

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* support for *--checkpoint=n*, which updates the .mc files every n hours
* fix bug for stuck fits on *--resume*: probabilities contain NaN
* better error message for missing store directory
* Python 3.8 support (time.clock no longer exists)

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* fix pickle problem for parameterized functions
* support multi-valued functions in Curve, shown with a coupled ODE example
* update support for newer numpy and matplotlib

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* --parallel defaults to using one process per CPU.
* --pop=-k sets population size to k rather than k times num parameters
* --resume=- resumes from --store=/path/to/store
* use expanded canvas for parameter histograms to make plots more readable
* use regular spaced tics for parameter histograms rather than 1- and 2-sigma
* improve consistency between values of cov, stderr and chisq
* fix handling of degenerate ranges on parameter output
* add entropy calculator using gaussian mixture models (default is still Kramer)
* vfs module allows loading of model and data from zip file (not yet enabled)
* warn when model has no fitted parameters
* update mpfit to support python 3
* support various versions of scipy and numpy