Created functions for rank-4, 3D tensor transposes (and corresponding unit tests).
The new functions are:
maths_fns.kronecker_product.transpose_12()
maths_fns.kronecker_product.transpose_13()
maths_fns.kronecker_product.transpose_14()
maths_fns.kronecker_product.transpose_23()
maths_fns.kronecker_product.transpose_24()
maths_fns.kronecker_product.transpose_34()