move non-thorn NRPyPN checkout into utils directory

[Roland Haas]

Right now NRPyPN is found in arrangements/EinsteinInitialData/NRPyPN but is not a thorn (or documentation directory or set of example parameter files).

At the time NRPyPN was proposed an alternative location in utils was metioned https://bitbucket.org/einsteintoolkit/tickets/issues/2417/include-pn-expressions-for-initial-momenta#comment-59057781 but apparently not used in git hash f31dd2f "einsteintoolkit.th: Add NRPyPN to EinsteinInitialData arrangement" of manifest

To avoid the confusion of having a non-thorn entity in the arrangements directory it may be good to move the checkout location of NRPyPN to utils. Eg ``utils/InitialData/NRPyPN`.