Rename quantity to make way for an actual quantity concept

[eggrobin]

No description provided.

[pleroy]

The phrase convertible_to_quantity is not overly elegant, and is what we have been calling "scalar" in the past. I am a tad worried that it's too much of a mouthful. For instance, it would be unpleasant to have to write:

template<convertible_to_quantity Scalar>
class ...

But let's see how things turn out once we actually have and use quantity

[eggrobin]

it would be unpleasant to have to write:

template<convertible_to_quantity Scalar>
class ...

But let's see how things turn out once we actually have and use quantity

We should never want to write that; our types are templatized on things that are actually quantity (that is, Quantity or double, not convertible_to_quantity, which includes int—and even char or bool…)