arxml: Only get min/max of the INTERNAL-CONSTR

[pempem98]

Under the tag DATA-CONSTR could have PHYS-CONSTRS and INTERNAL-CONSTRS in one DATA-CONSTR-RULE and could be have more than one tag DATA-CONSTR-RULE, with current logic, we only use the LOWER-LIMIT and UPPER-LIMIT of the tag INTERNAL-CONSTRS so I modify a little to make it get the lowest internal constr value and biggest internal constr value to lower/upper variables

[pempem98]

This may fix the issue https://github.com/ebroecker/canmatrix/issues/740 But still need someone do the verification

[ebroecker]

Hi @pempem98, I'll merge this one. If you have further improvements, just create a new pr!