(Partial) tracing of repeated indices on a single index_notation node

[campsd]

Add functionality to perform (partial) trace operations on repeated indices of a single index_notation node.

Examples:

A = tensor('A', 8, 8)
B = tensor('B', 2, 3, 4, 3)
i, j, k = indices('i, j, k')
traceA = A[i, i]
partial_traceB = B[i, j, k, j]

This could be implemented at the level of the index_notation node.