utils

- code is now generic (even if more testing remains).
- data types are now check accordingly to catch error in a clean way

In some cases (generally?) data types can only be checked at
runtime. This is the case for instance if function definition depends
on the dimension (which is likely to happen in many cases). Thus a
special procedure has been developed to indicate where the semantic
error occurs at runtime. (this is in some way related to issue #19)