Python summary function

[bob-carpenter]

Model: numba                     Total draws: 2668
variable       mean  mcse     std  5%  50%  95%    rhat   ess
alpha[1]    -0.0044        1.0014                1.0001  2008
alpha[2]    -0.0037        1.0006                1.0029  1769
Wall time: 2.19 ms

• model name or other tag at top
• total number of draws at top (it doesn't vary by variable)
• right align column headers
• truncate ESS to integers
• I like alpha[1] more than alpha.1, but would also be OK with latter as it's fewer characters
• It would be great if we could control significant digits in the call
• It would also be nice if we could control which quantiles are printed
• An alternative to 95% would be Q.95

Another alternative is to introduce a dependency on pandas for the summary, populate a pandas table, then print that. It takes care of things like truncating when you have 10,000 variables and also aligning, significant digits, etc.