Correct definition of nu and assign lower value for sigma_low (#459)

* Correct definition of nu and assign lower value for sigma_low

- 1 should be added to nu_minus_one using pm.Deterministic,
  otherwise addition of 1 in old definition has no effect
  and hence probability was non-zero for values of nu that
  are less than one
- Added definition and figure panel for log10(nu) to better
  align with Kruschke 2013
- Lowered sigma_low value to 10^-1 because sigma values less
  than 1 are credible (as seen for Group 2 Std Dev in Fig 3
  from Kruschke 2013)

With suggested changes, results will more closely align with
Kruschke paper, which example is based on.

* address review comments

Author: hyosubkim