String conversion and I/O

[snsinfu]

String conversion and I/O via std::istream/ostream should be supported by default. Additional support of the fmt library would also be good.

The string representation should not contain dimensional information. That just clutters output. The formatting should be the same as that of the underlying number type. Vectors and points should simply be formatted as space-separated numbers:

dim::vector<double, dim::mech::force, 3> force{1.2, 3.4, 5.6};
std::cout << force << '\n'; // "1.2 3.4 5.6\n"