v0.3.1 Reduce dependencies, addition of some continuous DPP samplers

@danielecc simplified installation instructions to work with minimal dependencies: numpy, scipy, matplotlib.
See also README for more details.
Additional dependencies:

• zonotope for the zonotope MCMC based sampler using cvxopt,
• trees for uniform spanning tree samplers using networkx,
• docs for the documentation using sphinxcontrib-bibtexand sphinx_rtd_theme,

can be installed locally after cloning the repo.

@guilgautier contributed with (see also /notebooks):

• an exact sampler for multivariate Jacobi ensembles used to do Monte Carlo integration
  On two ways to use determinantal point processes for Monte Carlo integration
  G. Gautier, R. Bardenet, M.Valko, NeurIPS, 2019.

• a Markov chain based sampler for beta-ensembles with polynomial potential
  Fast sampling from beta-ensembles
  G. Gautier, R. Bardenet, M.Valko, arXiv preprint, 2020.