Add a method Entity.as_dict().

[RKrahl]

It may occasionally be convenient to get a mapping with an entity object's attributes. For instance, one may do nice things like string formatting:

>>> inv
(investigation){
   createId = "simple/root"
   createTime = 2018-02-05 11:30:14+00:00
   id = 3
   modId = "simple/root"
   modTime = 2018-02-05 11:30:14+00:00
   endDate = 2012-08-06 01:10:08+00:00
   name = "12100409-ST"
   startDate = 2012-07-26 15:44:24+00:00
   title = "NiO SC OF1 JUH HHL"
   visitId = "1.1-P"
 }
>>> "%(name)s-%(visitId)s" % inv.as_dict()
'12100409-ST-1.1-P'