tensorproduct - Multiplication of multidimensional arrays

Product of multidimensional matlab arrays following Einstein summation convention, where repeated indices sum over

Implementation of Einstein summation convention for multidimensional matlab arrays, where repeated indices sum over.

Supports multiple:
- Outer products
- Inner products
- Singleton dimensions
- Pages

Example:
A = rand(5, 1,4,8); B = rand(4,10,5 ); R = tensorproduct('jzgi',A,'gxki',B,'kjg');
Outer: 'i','j', Inner: 'k', Page: 'g', Singleton: 'x','z', size(R) = [10,1,5,8].

Inputs:
- ind_R : string with indices of the output
- A : (non-empty) (multidimensional) matlab array
- ind_A : string with indices of array A
- B : (non-empty) (multidimensional) matlab array
- ind_B : string with indices of array B

Version compatibility: This implementation makes use of Matlab built-in function pagemtimes,
introduced in Matlab version R2020b. To make use of this implementation in previous Matlab releases, comment/uncomment few lines in the body of tensorproduct_core, as specified in the routine.

The performance of tensorproduct can be assessed by calling the function tensorproduct_benchmark.