[CDADT-018] In IVL<PQ> datatypes, the low and high values SHALL use comparable units
[CDADT-019] In IVL<TS>, the width/@unit SHALL be from mesure of time values (d, min, etc)
[DTITS-016] In PIVL_TS datatype, unit attribute of period element SHALL have a value from the time units
[CDADT-037] In PQ datatype, the unit attribute SHALL be from UCUM code system