Reimplementation of vex::Filter::Env
Reimplementation of vex::Filter::Env

Uses vex::Filter::General internally, adds support for OCL_TYPE
environment variable (#62).

Poulpy21/vexcl,Poulpy21/vexcl,kapadia/vexcl,Poulpy21/vexcl,ddemidov/vexcl,kapadia/vexcl,ddemidov/vexcl