Semantics for select
initial back end for select
using a names vector as requested in #232
Use new select implementation everywhere.
Closes #232. Closes #192. Closes #172. Closes #158
Replace use of var_index
Describe select in NEWS
Tweak news formatting
Fix titles in news
Add tests for select
More documentation
Better implementation of num_range.
Thanks to @wch
Add tests for num_range