Add local coordinates

[ettersi]

It would be great if CoordRefSystems could implement the East North Up local coordinate system.

A major design issue when it comes to local coordinates is whether such coordinates should remember their origin. I would like to propose that in CoordRefSystems local coordinates should remember their origin but the cstrip() function (see #47) should return only the offset, i.e.

x = Cartesian(1,0,0)
y = Cartesian(1,1,0)
@assert cstrip(ENU(x), y) == SVector(1,0,0) # y is one unit to the east of x

This would allow local coordinates to represent a unique point in space just like their global counterparts, and at the same time it would allow the user to easily "relocate" a local coordinate using cwrap(ENU(y), cstrip(ENU(x), local_coord)).

[juliohm]

I remember seeing this nice idea in MapMaths.jl, where you can add angular coordinates to local coordinates in meters to shift a point over a wrapped segment over the sphere.

We plan to add an operation for this instead:

CRS + ENU ---> CRS

It doesn't need to be the + function, it can be a specific name such as shift or move. We will see which properties this operation satisfies before we commit to a particular name/design.

Would that address your original issue?

[ettersi]

Addition would work well this way. How would you handle subtraction or convex combinations? I think defining e.g. CRS - CRS -> ENU is problematic, because some people might instead want CRS - CRS -> WrappingENU or CRS - CRS -> Cartesian (i.e. the difference in an ECEF coordinate system).