gtCheckParameters : updated finally the running version of checking parameters. Possibility to check type of parameters ('default','used','extra','all') using checkType argument.
                    section_name must be field of parameters
gtModifyStructure : now it handles also usertype and has the freedom to add/remove additional fields to the structure (giving negative usertype)
                    Added some functions to perform the check.

Signed-off-by: Laura Nervo <laura.nervo@esrf.fr>