BUG: Zero Mass Flow Tank causing Empty Velocity Data

[phmbressan]

Pull request type

• [X] Code changes (bugfix, features)

Checklist

• [ ] Tests for the changes have been added (if needed)
• [X] Lint (black rocketpy/ tests/) has passed locally
• [X] All tests (pytest tests -m slow --runslow) have passed locally
• [X] CHANGELOG.md has been updated (if relevant)

Current behavior

Adding a Tank with zero mass flow rate at all times causes an error on exhaust_velocity computation.

New behavior

Zeros were added as defaults to the exhaust_velocity calculation and the total_mass_flow_rate formula as changed to an identical one that avoids possible zero divisions errors.

Breaking change

• [ ] Yes
• [X] No

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.46%. Comparing base (3a4c742) to head (1a16b14). Report is 44 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #677 +/- ## =========================================== - Coverage 75.87% 74.46% -1.41% =========================================== Files 85 89 +4 Lines 10085 10368 +283 =========================================== + Hits 7652 7721 +69 - Misses 2433 2647 +214 ```

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