IMAP-Lo: Skystrip L3 products (Phase D deferred)

### Summary

5-7 day "skystrip" products for L3 (per mapping product spreadsheet)

### Requirements

The more specific the better.
  - [ ] SP-corrected s/c frame
  - [ ] SP-corrected heliospheric frame
  - [ ] SP-corrected heliospheric frame spectral index


### Related tickets

Depends on L2 skystrips

### Estimate
32 pair-hours (old estimate though!)