mc_tools: simplify measure_aux

- for pieces that need to be precomputed for several
measures.
- put them under shared_ptr, and register then with add_measure_aux.
- they must be callable, as void ().
- TODO : add this in the doc when tested
gfs: mesh product, rename shape method...

- just a renaming for readibility.
arrays: correction of value_type of matrix_expr

- forgot to correct the value_type of matrix_expr, and vector_expr
as was done long ago for arrays...
- also added cases for arrays until dim 10
- TODO : replace this trait in arrays with a tuple tools for any dim..
  not urgent.