Fix/matrix multiplication row major

[McModknower]

Due to base::blas_uninit::axcpy_uninit not taking the stride into account for calculating the number of elements at

nalgebra/src/base/blas_uninit.rs

Line 114 in 3320ecc

array_axc(status, y, a, x, c, rstride1, rstride2, x.len());

, multiplying matricies with a row stride other than 1 fails with the following error message:

thread 'core::blas::array_axcpy_nonstandard_stride' (21450) panicked at /home/teddy/uni-notes/ma/rumspielen/nalgebra/src/base/blas_uninit.rs:71:19:
unsafe precondition(s) violated: slice::get_unchecked_mut requires that the index is within the slice

This indicates a bug in the program. This Undefined Behavior check is optional, and cannot be relied on for safety.

This pull request introduces a test for this case and fixes the bug by properly calculating the number of elements via a tested helper function number_of_elements(len: usize, stride: usize) -> usize.