Cube support

[Jimmacle]

This PR implements #1938. It depends on the changes in npgsql/npgsql#3867.

This is currently pretty rough and incomplete but I'm putting it up as a draft for visibility. It's my first dive into working with EF Core database provider implementations so feel free to point out anything I'm doing wrong.

Current API as of e130ecd:

PostgreSQL Operator/Function	NpgsqlCube API
cube && cube	bool Overlaps(this NpgsqlCube a, NpgsqlCube b)
cube @> cube	bool Contains(this NpgsqlCube a, NpgsqlCube b)
cube <@ cube	bool ContainedBy(this NpgsqlCube a, NpgsqlCube b)
cube -> integer	double NthCoordinate(this NpgsqlCube cube, int n)
cube ~> integer	double NthCoordinate2(this NpgsqlCube cube, int n)
cube <-> cube	double Distance(this NpgsqlCube a, NpgsqlCube b)
cube <#> cube	double DistanceTaxicab(this NpgsqlCube a, NpgsqlCube b)
cube <=> cube	double DistanceChebyshev(this NpgsqlCube a, NpgsqlCube b)
cube_union(cube, cube)	NpgsqlCube Union(this NpgsqlCube a, NpgsqlCube b)
cube_inter(cube, cube)	NpgsqlCube Intersect(this NpgsqlCube a, NpgsqlCube b)
cube_enlarge(c cube, r double, n integer)	NpgsqlCube Enlarge(this NpgsqlCube cube, double r, int n)
cube_dim(cube)	int Dimensions
cube_ll_coord(cube, integer)	double LlCoord(int n)
cube_ur_coord(cube, integer)	double UrCoord(int n)
cube_is_point(cube)	bool Point
cube_subset(cube, integer[])	NpgsqlCube Subset(params int[] indexes)

Looking for feedback on these points:

• Need a better name for NthCoordinate2 that differentiates it from NthCoordinate. Maybe shorten Coordinate to Coord to be consistent with LlCoord and UrCoord?
• Implementing indexer translation (or more) for NpgsqlCube.LowerLeft and NpgsqlCube.UpperRight or only supporting the LlCoord and UrCoord methods.

[roji]

@Jimmacle just to set expectations, since this depends on npgsql/npgsql#3867, it would be better to wait for that to be merged before spending too much time on this. In addition, in the next 2 months (more or less) I'll mostly likely be very busy with Npgsql/EF/.NET stabilization work, so there's little chance I'll be able to give this much attention...