Construct input for _LLMatrix routine as zeros
Correct import of numba wrapper
Don't make copies of data when slicing

We can construct a new Waveform using the getitem function (by
indexing the Waveform object).  Previously, I made copies of all such
data.  However, this requires allocation and copying time for
something that really only gets used, rather than written to, in
principle.  So, to conform with the slicing behavior of arrays, we
don't make copies of the data.