Pdarray indexing

[jeremiah-corrado]

Implement multidimensional pdarray indexing.

For example, the following indexing patterns now match numpy's behavior, where A is a 3D pdarray and a is a 1D pdarray:

x = A[a, a, a]
x = A[a, a, 1]
x = A[a, a, :]
x = A[a, 1, 1]
x = A[a, :, 1]
x = A[a, ...]

Also implements rank expansion via 'None' indices. For example, the following would return a 4D array, where the 3rd dimension has a size of 1:

x = A[1, :, None, a]

*also fixes a bug in unstackMsg

[bmcdonald3]

Looks good, couple of questions.