Clean up test pair class definitions a bit

In particular introduce define an intermediate class testStandardPair
that holds all the standard methods but does not require particles to
live on the same structure.
Change order of initialization in MixedPair1DStatictestShell
Same class reorganization for the pair classes
Extend Disk.project_point() method to the case in which the position
is in the plane of the disk, not 'above' it
Minor fixes
Fix structure functions for Plane->Disk case