merge for non-finite matrices; new functions to collate files

This pull request has two main features. They got onto the same branch by carelessness on my part. Please don't merge this yet. I still need to debug the second feature below.

1. the fix for non-finite step sizes (I did the merge, but decided to keep my code.)
2. a new class ConcatGeneral to concatenate ngmix files from arbitrary job layouts

The second item above is meant to support processing with ngmix over the MEDS files not done at BNL. It allows the user to feed in a simple text file with the job layouts and then the ConcatGeneral class will read in the chunks and do what it does normally.