Changed type of "validate" param from init list to vector
Removed warnings for optional attributes in validation of SimpleTag
Extended checks for valid SI units to handle vectors of strings
Slight changes in unit checks code towards more oop style
Added checks for units being of valid SI type in DataTag
Added checks for unit compatibility between tags & references

New checks for the compatibility between units of (simple or data) tags and their referenced DataArrays' units and implemented their use in DataTag.