glucose matrix inversion

>> inversion

This inverts the glucose matrix, computing spatial distribution of
glucose for a given K matrix.

>> testing

The key difficulty is converting from N x N matrix (indices i, j) to N^2
x 1 (index c) vector, so the corresponding maps IJtoC and CtoIJ are
extensively tested against solutions obtained independently. testing
should print all ones (= true) otherwise the test is failing.

The matrix inversion (last step of the program) comes with some trouble
and needs more work but the other weeks seem stable.