mtlb_imp Matlab colon emulation function

## Description

Matlab and Scilab colon behave differently in some particular cases:

- With empty matrices: The `:` operator must be used with scalars in Scilab and gives an error message when used with empty matrices while Matlab returns [] in these cases.

The function `mtlb_imp(A,B[,C])` is used by `mfile2sci` to replace `A:B[:C]` when it was not possible to know what were the operands while porting Matlab code to Scilab. This function will determine the correct semantic at run time. If you want to have a more efficient code it is possible to replace `mtlb_imp` calls:

- If `A`, `B` and `C` are not empty matrices `mtlb_imp(A,B[,C])` may be replaced by `A:B[:C]`

Caution: `mtlb_imp` has not to be used for hand coded functions.