some issues in IntervalEvent

[Sergio Miranda Freire]

There are some arguments missing in its initializer (those inherited from Locatable and from Event). It is also necessary to call the superclass initializer.

Method timer is not correctly implemented. It should return a DvDateTime corresponding to the value of attribute time subtracted from the value of the attribute width.

There is no check for the validity of mathFundtion value.
Math_function_validity: terminology(Terminology_id_openehr).has_code_for_group_id(Group_id_event_math_function,
math_function.defining_code)

Here we will have to use the internal openehr terminology service