Corrections to the PAN CrIS-JPSS1 TROPESS forward stream to correct climatology

The forward stream is updated in real time, meaning that discontinuities can be introduced as new versions are rolled out. This repository corrects for two of these updates in PAN: 1) an error in the a priori climatology of PAN, and 2) updates to the climatology of CCL4 and CFCs. In addition to this, the forward stream should not be used prior to May 1, 2022 as there are discontinuities that we do not know how to explain. We also note a small (0.01 to 0.02 ppb) positive bias between the JPSS1 forward stream and the retrospective SNPP stream.