fix type of a lookup table/vector for channel order -> index

Investigated compile error with @reister. Changing the type of the variable whose entries are used as indices to Eigen::Vector3i solves the problem. Using Eigen::Matrix<size_t, 6, 1>, which seemed even more suitable, did not work.

requested review from @reister

assigned to @daab

mentioned in issue sw/simox/simox#66

mentioned in merge request sw/simox/simox!117 (merged)

mentioned in merge request sw/armarx/meta/axii!491 (merged)

approved this merge request

merged

mentioned in commit 0d3abb06

Thanks @daab !