BUG: Correct importing behavior for Categoricals in StataReader

Ensure that category codes have the same order as the underlying Stata data.
Also adds a flag that allows categorical data to be treated as ordered or
unordered when importing.
Merge pull request #8836 from bashtage/stata-monotonic-categoricals

FIX: Correct Categorical behavior in StataReader
BUG: DataFrame.stack(..., dropna=False) with partial MultiIndex.