[QUANTITY] Data Model for Quantity v0.5

[Jonathan McDowell]

Brian,
 You make some proposals for changes to Q.

 (1)  You propose adding Accuracy to BasicQ. 

  I don't like this for the same reason I don't like mappings in BasicQ:
  the idea behind BasicQ was to make something really simple that
  would not be much more than a name and a value and a unit (which
  we can currently stick in a single FITS keyword). We added the
  full Frame for various technical reasons. But I would prefer
  not to add anything else. Otherwise, the distinction between
  a BasicQ and a CoreQ becomes less useful.

 ".. you have to allow the user the option to record the errors".

  We do, the option is "Use a CoreQ". But the reality is that
  there is a LOT of data out there with no known errors and
 actually, the more I think about it, a lot of metadata that should
 not have errors (CCD chip number, pixel position of a hot pixel,
 (arguably) proposed RA and Dec of pointing, etc.). Certainly
  the majority of integer and string metadata. Possibly some
  floating point metadata representing requested values (although
 I guess they could come with tolerances, they usually don't.)
 And indeed, 99 percent of all FITS header metadata out there right
 now is error-free for better or worse. 
 
 (2) You want to change add/remove/getValueQuantity
                     to add/remove/getAlternativeValue.
     
  I don't have a big problem with this.
  I'm not so keen on removing the getCurrent... but if others
  want to I'll go along with it.


OK, bedtime.

 - Jonathan