Update default Moon shapemodel

**Description**

The current default lunar shapemodel in ISIS is... not even remotely the highest quality that is available these days. It's a 128 px/deg DEM from LOLA data only (from very early in the LRO mission, I believe), and has pretty significant interpolation artifacts everywhere but the poles.

The best shapemodel available for the Moon these days is the 512 px/deg SLDEM, which has almost no interpolation issues, and truly does resolve features at that scale. I'm not suggesting that you include the whole 64GB for a global DEM at 60 m/px, but a downsampled version at 128 px/deg would still be a dramatic improvement over the current DEM, and take up no extra space.

**Processing**

There are a few complications in actually making this DEM:
- The SLDEM only goes to 60° N/S, so you need to splice in LOLA polar DEMs. Fortunately, the most recent 60m/px LOLA polar DEMs go out to 60° N/S, and have no perceptible artifacts.
- If you're enthusiastic, you can also splice in the Improved LOLA south pole mosaic, which probably won't be detectable at these resolutions.
- The current built-in DEM is reduced in size by storing it as 16-bit with a base and multiplier, and makes it reasonably nice to read outside ISIS by using a base of 1737400 (mean lunar radius) and multiplier of 0.5. This quantizes the DTM a bit more than the [32-bit data type does](https://github.com/DOI-USGS/ISIS3/issues/4604), but is reasonable at this scale (the smallest multiplier parameter we could use is 0.33 without truncating the highest points on the Moon, and 32-bit floating point at the lunar radius is the same quantization as a multiplier of 0.25). *Making* that scaling is a bit tricky, but the following suffix does it: `+16bit+1721024:1753783.5`.

Alternately, if this idea is accepted, I have a merged SLDEM+LOLA poles DEM pre-made at 512 px/deg that I can downsample and convert to 16-bit and post somewhere.

Locate the current built-in lunar DEM and the code or data configuration that loads it. Review the proposed SLDEM plus LOLA polar coverage and the 16-bit scaling suffix, then determine how the replacement can be generated and packaged at 128 px/deg. Done means the new default model is available without extra space and preserves the stated lunar-radius scaling behavior.

Written by the indexing model from the issue text.