Add copy assignment operator to `strided_bounds`
Add copy assignment operator to `strided_bounds`

`strided_bounds` defines copy constructor, but lacks copy assignment
operator

kernhanda/GSL,Josaphat/GSL,matus-chochlik/GSL,bashrc-real/GSL,neilmacintosh/GSL,vladon/GSL,jbcoe/GSL,garyfurnish/GSL,martinmoene/GSL