124 adding .err and .png to gitignore

Fixes #124.

Review Checklist for Source Code Changes

[x] Does pip install still work?
[ ] Have you written a unit test for any new functions?
[x] Do all the units tests run successfully?
[x] Does adler run successfully on a test set of input files/databases?
[x] Have you used black on the files you have updated to confirm python programming style guide enforcement?