FIX: Enable fixed width strings to be read from Stata 13 (117) files

Fixes a bug which prevented files containing fixed width string data from being read.  Stata 13 files also allow variable length strings, which are not supported in the current version, and an explicit exception regarding this type is now given.  Added tests which cover these cases, and Stata 13 format files. fixes #7360
FIX: Stata writer no longer ignores encoding when writing strings

The encoding was not used when writing strings. Fixes #7286
Merge pull request #7450 from bashtage/stata-13-strings

FIX: Enable fixed width strings to be read from Stata 13 (117) files