prefetch field

	 A 'prefetch' attribute on a field makes the server look up the value on the
	  pentominos server, using the current patient id, the prefetch field as the
	  raw request. The latest returned value is the only one used.
	 A prefetch field overrides the value field.
	 If a prefetch fails due to no data available, the value field is used.
	 


Things to think about:
 - What to do with resumes in included macroes. Append/prepend to main resume?
 - How to integrate fetched values (eg. from Pentominos) and how to store where
   the values came from (device/usertyped)
 - Can connection keep alive be used for anything useful? Session? State?
 - What to do about commits on nondata macroes? (eg. start)