Refl1d

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* add `--checkpoint=n` to save mcmc state every *n* hours.
* hide resolution bars using *Probe.show_resolution = False* in your model.py
* fix residuals, fresnel, logfresnel, and q4 plots when interpolation is used
* fix error contour save function: rhoM was not being written
* fix simulation error: theory and simulated data were sharing the same vector
* fix doc generation: now builds with sphinx 2.4.0 and matplotlib 3.1.0.
* python 3.8 support requires bumps v0.7.14 (released 2020-01-03)

**BREAKING CHANGE**: old-style data loader with sample_broadening set

* sample_broadening was applied twice: once to the base Δθ from when the probe
was created and again whenever ΔQ was computed; this is not a problem with
the new *load4* function. The code was fixed, which may cause difficulties
when reloading old fits.

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* json save: material and magnetism are now json objects
* json save: include non-parameter data in save file
* load4: accept 2 and 3 column data
* load4: override resolution and uncertainty given (or missing) from file
* load4: accept multi-entry data as Q probe without knowing theta/lambda
* load4: set the data slice to load
* load4: set default radiation to 'neutron'
* allow fit by number density for materials
* fix interpolation when plotting reflectivity between measured data points
* fix bug in dQ when sample_broadening is initialized to non-zeros
* revised installer: embedded python in zip file
* functions to compute transmission and reflection at each layer
* allow simulation with uncertainty from data
* force minimum uncertainty in data to 1e-11
* change default data view from Fresnel to log10
* apply resolution to saved Fresnel curve
* improved python 3 support

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* fix json save for MixedExperiments
* save smooth magnetic profiles
* fix abeles code to choose correct branch cut below critical edge
* force absorption to be 0 or positive

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* make sample broadening a fittable parameter
* allow model + data to be loaded from zip file (bumps 0.7.12 and up)
* improve serialization support

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* added serialization support
* added option to supply uncertainties when simulating data

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* fix for plotting spin asymmetry when data is not present (model-only)
* added requirements to setup.py so that `pip install refl1d` suffices