Remove deprecation warnings.

[stscirij]

• Remove references to err_gehrels.
• Clean fpavg code so it doesn't divide by zero when calculating the conversion from counts to flux

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8eb63d1) 87.64% compared to head (f737821) 87.41%.

Files	Patch %	Lines
tests/helpers.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #235 +/- ## ========================================== - Coverage 87.64% 87.41% -0.23% ========================================== Files 17 17 Lines 1392 1367 -25 ========================================== - Hits 1220 1195 -25 Misses 172 172 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[stscirij]

Developer pull request test run here: https://plwishmaster.stsci.edu:8081/job/RT/job/Calcos-Developers-Pull-Requests/6/

[nden]

Should numpy be limited to < 2.0 before this release? Otherwise lgtm.