added new quantity function to return astropy quantity, all other functions now use this function to get the value or unit, trying to preserve api
update to use Quantity, getting warning from astropy still
updated _si to use astropy constant objects

also fixed bug in wcs introduced by using new constants
Removed models from constants, created new module
fixed some errors, updated print_all(), updated doc strings
updated doc string, removed units
fixed failing test in wcs due to new constant object
Fixed #617
Merge pull request #661 from Cadair/norm_fix

Fix Map.norm
Merge pull request #657 from ehsteve/goes

Fixed #617
Merge remote-tracking branch 'upstream/master' into sun
Merge pull request #659 from Cadair/HMI_fix

Fix a bug in HMIMap
Updated changelog
Merge pull request #656 from ehsteve/sun

Update to sun.constants to use astropy constants object
First attempt at adding some support for IRIS

This is a pretty basic attempt, it adds a function that converts a
SJI cube into a MapCube, by replicating the header.

Also added a 'blank' map source for this type of image.
Add IRIS to map sources __init__
Change class doc string!
Add IRIS to changelog