[variational] Update default parameters in ``run()``
[markov_chain] no op for run(0)
[markov_chain, doc, examples] Add mc.clear() function

remove changes to doc/user_guide.rst
[gauss] faster determinant, better comments
[markov chain] Reorder adapt method, improve comments

Motivation: if covariance matrix invalid, allow better exception
handling from the user
[markov_chain] increate adapt_count before proposal update
(wip) Drop support for covariances in R value
[r_value] fix unit tests
[examples/r_value] Covariance -> variance in
[doc] section "How to use this documentation": pypmc --> ``pypmc``
[tools] add parallel_function
[examples] add "ultimately_parallel.py"

o parallel sampling from a parallel likelihood function
[parallel_function] improve documentation
[mixture.pyx] start docstrings with newline such that cython correctly includes the call signature
[doc] correct reference [Wra+09] -> [Kil+09]; [Wra+09] explains PMC, not adaptive MCMC
[doc] "calculates ..." -> "calculate ..."
Revert "[markov_chain] no op for run(0)"

This reverts commit 69f811d594f6a60e400ba917a094fb533e4220e2.

raise an error if sampler shall produce a negative number or zero samples
[doc] function r_value: line break for better display
[markov_chain] remove todo comment
[ImportanceSampler] introduce "clear()" function like for "MarkovChain"