Allow SpatialUserObjectVectorPostprocessor to get points from UO

[aprilnovak]

Reason

The SpatialUserObjectVectorPostprocessor lets you specify a number of points at which to output the value of a spatial user object. For the spatial user objects of type NearestPointLayered..., there is one unique value of the user object in num_points * num_layers "bins." For instance, if the user object is defined like this:

[UserObjects]
  [npla]
    type = NearestPointLayeredIntegral
    direction = y
    num_layers = 3
    variable = u
    points = '0.375 0.0 0.3
              1.125 0.0 0.3
              0.375 0.0 0.9
              1.125 0.0 0.9'
  []
[]

this will result in 12 bin integrals (4 points times 3 layers). If you wanted to get the value for each user object in the SpatialUserObjectVectorPostprocessor, you'd need to provide:

[VectorPostprocessors]
  [spatial_manually_provided]
    type = SpatialUserObjectVectorPostprocessor
    userobject = npla
    points = '0.375 0.25 0.3
              0.375 0.75 0.3
              0.375 1.25 0.3

              1.125 0.25 0.3
              1.125 0.75 0.3
              1.125 1.25 0.3

              0.375 0.25 0.9
              0.375 0.75 0.9
              0.375 1.25 0.9

              1.125 0.25 0.9
              1.125 0.75 0.9
              1.125 1.25 0.9'
  []
[]

Writing a script or manually typing out these points is cumbersome. In Cardinal, we have a spatial user object that defines the "nearest point" bins based on a subchannel discretization, which is even more tedious to provide the points to the SpatialUserObjectVectorPostprocessor because the points themselves are never provided explicitly to the spatial UO.

Design

Add a UserObject::spatialPoints() interface that provides points that the SpatialUserObjectVectorPostprocessor can use instead of the user providing something via points or points_file. For the NearestPointLayered... user objects, this interface should provide the points in the above spatial_manually_provided VPP example.

Impact

Big improvement in usability for spatial user objects in Cardinal. New feature.