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. 
I saw this in earlier comments too: why have we changed this to DOSING_TYPE_SIMPLE? Wouldn't it read more clearly as "...is null for SimpleDosingInstructions"?