Add optional reason attribute to ISM_TRANSITION class

Changes made in Release-1.0.3-TB branch of specifications.RM repo.

I agree on Ian's suggestion about making the reason as 0..* DV_TEXT.
This makes it possible to add several coded reason attributtes. This could be useful in some clinical use-cases.

IM: needs to be 0..* DV_TEXT - current medication action - different reasons why med is cancelled etc.