generalise indexing #77
Description
At the moment only indexing of dense 1d vectors is supported, should generalise this:
- allow dense
nd indexing - allow diagonal indexing
- allow indexing of sparse or diagonal tensors
e.g.
a_ij { (0:3,0:3): 1.0 }
b_ij { a_ij[0:2, 2:3] }
b_i { a_ij[0..2, 2..3] }