lua/numeric: revise multi_index_to_offset, offset_to_multi_index

Use 1-based index for both functions and call multi_index_to_offset
correctly.
lua/utility/numeric: document module

This commit refs #115280
add unit test: lua/utility/numeric/multi_index
excluded_volume: fix test, check code snippet in documentation

The test didn't do anything but defining a function (which was left
unevaluated).

Add a test for the code snippet quoted in the documentation. The code is
inlined from the test script.

For the second point, the test scripts need to be copied to the binary
tree and the documentation becomes dependent on the whole tree test/lua.
This is a disadvantage since modifications of a test script will trigger
the rebuild of the whole documentation. The best solution is probably to
add a CMake switch which allows one to not build the documentation.