lua/numeric: fix 1-based index in multi_index_to_offset

multi_index_to_offset assumed to receive a zero-based multi index.

This fixes the failing unit test for multi_index.
test/multi_index: make the test work for any dimension
lua/…/numeric: revise offset/multi-array conversion

Use simple loops in favour of the recursions, which leads to much
simpler (and more efficient) code without helper functions. Actually, I
wondered whether the C++ counter part of these functions benefits from
the recursions or not.

doc: fix UTF-8 character for \times in "n×n matrix". The previous
     character looked the same but was unknown to LaTeX.

test: reduce amount of default output