Temporary multivector expressions are not supported for MSVS

Trying to compile an example with temporary created from a multivector
expression leads to crash of VS2010 (C1001 internal compiler error) and
compilation error in VS2012.

If one really needs multivector expression temporaries, those may be
replaced with tied vector expressions:

    auto tmp = vex::make_temp<double>(mv);
    mw = tmp;

is equivalent to

    auto tmp0 = vex::make_temp<double>( mv(0) );
    auto tmp1 = vex::make_temp<double>( mv(1) );
    mw = std::tie(tmp0, tmp1);

where mv and mw are two-component multivectors.

refs #50