add sample summary

[andymiller]

we have names of variables and collections of state samples - it'd be nice if we could have a very generic method for summarizing them on the command line (and maybe a method for pretty printing them in a notebook). Something like

variable  mean  sd  skew    autocorr    eff_samp_size
   x      0.22
   y0      0.21
   y1      0.11
...
   y100  0.01  ...

where x is a scalar, and y is a vector.