TargetType should be defined in datamodel.txt and headers

Define the values of the TargetType enum in datamodel.txt along with a new enum, FiberStatus.

Also write this definitions in all the relevant product headers (pfsObject, pfsMerged etc) so they are self-describing.

The TargetType enum should contain the following values:

SCIENCE
SKY
FLUXSTD
UNASSIGNED
ENGINEERING

The new ENGINEERING value is to accommodate data where the engineering fibers are illuminated. Note that the fiber characteristics (broken, blocked etc) have been moved. They are now in a new enumeration, FiberStatus (or better name), with values:

GOOD
BROKENFIBER
BLOCKED
BLACKSPOT
UNILLUMINATED

The value BROKENFIBER replaces BROKEN (which is confusing as 'BROKEN' could indicate a broken cobra or a fiber). A perfectly functioning fiber is labelled with FiberStatus==GOOD.