Move `Domain.dx` to `OnGrid.dx`

[astanziola]

The .dx attribute only make sense for fields defined on a grid, and can be uniquely found from the shape of OnGrid.params and the size of the domain.

It is much more natural to define the domain as

domain = Domain(size: tuple)

# Or, potentially
domain = Domain.from_grid(N, dx)

It would be even better to define domains that are not rectangular, for example

domain = Domain()  # Abstract domain

class RectangularDomain(Domain):
  L: tuple

class SphericalDomain(Domain):
  R: float

which then allows to define non standard discretizations. For example, Continuous can work on arbitrarily shaped domains, FourierSeries probably only makes sense on RectangularDomain while something like a SphericalFourierSeries (see for example s2fft) could be implemented on a SphericalDomain.

This is clearly a breaking change.