Support for alternate KB parameters

Updates to KB parameters

• Previously, using KB orders other than 0 and KB widths other than 2.34 were not possible - this is no longer the case.

Updates to initialization

• Initialization has been harmonized. The code should now cleanly determine whether the input a float or a tuple for parameters such as 'order' and then check input dimensions prior to building interpolation tables.

Printing models

• __repr__ commands have been updated for all models, so it should be possible to print(ob) and get a brief summary of that object's attributes instead of a smorgasbord of PyTorch registered buffers. Examples of this have been added to the Jupyter Notebook examples.

Refactoring of nufft utilities

• nufft_utils has been moved into the nufft submodule where it more belongs. Currently torchkbnufft itself will import this and alias it to the old nufft_utils location, but eventually this will no longer be the case.