Use bottleneck to calculate nanmean in statistics function

[gsheni]

• pandas docs mention this:

bottleneck: for accelerating certain types of nan evaluations. bottleneck uses specialized cython routines to achieve large speedups. If installed, must be Version 1.2.1 or higher.

• We can use bottleneck to calculate the nanmean, and speed up our describe function
• We should make this an optional dependency
• It seems that we might just need to install bottleneck and we should be able to get the speed improvement, but we should verify this.