TRUNK-4445 Added support for custom dosing type for DrugOrders. Changed ...

...the current dosing type enum to a string, while keeping the support for SIMPLE and FREE_TEXT dosing types using constants.
DrugOrderValidator also now takes care of checking for SIMPLE, FREETEXT and null dosing types, any custom dosing types would require their own validation. 
This is a setter, which can no longer work right since its input is a boolean, but now there are multiple possible dosing types. I strongly suggest we remove this method (and the getter) so that anyone whose code is trying to use these fails early and loud, rather than having errors sneak through.