Add division by a scalar

[mpusz]

Right now the library seems to not support the division by a scalar. For example:

fs_vector<si::length<si::metre>, 3> v = { 4q_m, 8q_m, 12q_m };
std::cout << v / 2q_s << "\n";

The expected output would be to print quantities of velocity types.

Multiplication by the inverse value of a scalar really does not work for all cases.

[BobSteagall]

This is on the schedule to be added to R7.

[BobSteagall]

Done for r7z