New docs, backend optimizations

These changes are in preparation for pygrank's third major release (0.3.0):

• The updated documentation can be found here: https://pygrank.readthedocs.io/en/latest/
• Backends now admit initialization parameters, such as preferred device and mode of operations. Additionally, GPU backends have a default mode of trying to use dense matrices to represent graphs, as long as those matrices reside in memory. This ensures that the fastest possible computations will be performed out-of-the-box.
• Backend bottlenecks have been optimized to require similar execution time as other fast computations.

Bug fixes and maintenance, improved documentation

0.2.10

sparse_mkl backend integration

0.2.7 - Archival release (fully covered code base)

optimized density and conductance to use fast computations and allow …