v0.3.0 Addition of vfx sampler for finite DPPs

@danielecc contributed with an implementation of the vfx sampler, the associated documentation and tests.

In practice

DPP = FiniteDPP('likelihood', **{'L_eval_X_data': (eval_L, X_data)})
DPP.sample_exact(mode='vfx')

See the corresponding NeurIPS 2019 paper of Derezinski, Calandriello, and Valko Exact sampling of determinantal point processes with sublinear time preprocessing