add cell spiking information rate and information 'specificity'

According to

Skaggs, W.E. et al., 1993. An Information-Theoretic Approach to Deciphering the
Hippocampal Code. In Advances in Neural Information Processing Systems 5, pp.
1030–1037.
add occupancy_prob_distrib (+ unit tests)
add missing doc to occupancy_prob_dist()
remove duplicated code from gridcells.analysis.info
fix handling of NaNs in the information rate functions
Merge branch 'information-rates'