Pyabc

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* Fix several errors with the readthedocs (rtd) documentation.
* Speed-up rtd build by removing unnecessary conda and pip requirements.
* Clean-up requirements for travis and rtd.
* Change rtd design from alabaster to sphinx_rtd_theme since it implements
better navigation.

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* ABCSMC can pass observed summary statistics to distance functions
(required for some scale functions, and to make the
methods robust to volatile summary statistics).
* Implementation of more scale functions (distance_functions.scales), in
particular some taking into account the bias to the observed data.
* AdaptivePNormDistance accepts a Callable as scaling scheme, allowing
for more flexibility.

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* Can specify kde and number of bins for all visualization routines.
* Can re-submit observed sum stats to ABCSMC.load() function in case
it cannot be read correctly from the db.

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* Fix serious memory problem resulting from pickling more than necessary
for parallel sampling.
* Update logo, readme.
* Make tidying optional in abc-export (default behavior not changed).

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* Minor error and warning fixes due to API changes in pandas, seaborn (not
used any more), and change of the R installation on travis.

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* Default visualizations like plot_kde_matrix() can plot reference values,
useful for testing purposes.