Einsum, initial PR (after rebase)

[HubertKrawczyk]

This PR introduces initial version of einsum operator to SystemDS.
Einsum operator follows structure of Numpy einsum https://numpy.org/doc/stable/reference/generated/numpy.einsum.html

For tests, R package einsum https://cran.r-project.org/web/packages/einsum/einsum.pdf is used

Implemented so far:

• validation of einsum instruction
• preprocessing: simplifying inputs by contracting dimensions that can be contracted at the beginning
• preprocessing: identifying diagonal of matrix
• plan generation
• plan execution using individual operands (matrixMultiply, multiply, transpose, sumR, sumC, sumAll)
• fallback: Cell template with for-loops for einsums that cannot be solved using further 2D summations (e.g AB,BD->ABD)

(M Thesis at TU Berlin "Efficient Einstein Summation in Apache SystemDS")

[mboehm7]

LGTM - thanks for this first version of the new einsum framework @HubertKrawczyk. I also appreciate the clean rebased PR. Overall, the code looks fine - during the merge, I only resolved a few warnings (imports, unused code, generic typing).