Update ALCDEF filter definition for our PanSTARRS filter set

[talister]

From Brian Warner on the MPML:

The [alcdef.org](http://alcdef.org) site and definition/standards docs have been updated to distinguish between Pan-STARRS and Sloan (SDSS) ugriz filters and magnitudes.

Use "PX" for Pan-STARRS where "X" is U, G, R, I, or Z (case-sensitive).
Use "SX" for Sloan (SDSS) where "X" is U, G, R, I, or Z (case-sensitive).

Color indexes follow suit, e.g., PGR, PIZ, SGR, SIZ.

The two systems are not exactly the same and have systematic offsets, i.e., not just a zero-point shift but also based on color index. The community (data user) decides where, when, and how the differences twixt the two are important and when they can be considered "close enough."

We should update our export routines to follow this