Textual representation of vector expression
Textual representation of vector expression

String representation of a vector expression terminal is obtained through call
to `partial_vector_expr<Term>::get()` function instead of hard-coding it inside
vector_expr_context.

refs #37

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