Run checkstyle on GitLab CI

See merge request !197

Fix checkstyle warnings

See merge request !196

Fixes #831

Add unit test for DSST orbit determination in backward propagation mode.

Closes #717

See merge request !199

Fixes #717

Issue 818

Closes #818

See merge request !198

Fixes #818

Allow backward DSST osculating event detection

Closes #798

See merge request !195

Previously it would fail, now event detection will work, albeit with
some errors due to #834.

Fixes #798

Trigger performance tests after snapshot build

See merge request !194

Update time toString() methods

Closes #637, #590, #591, and #825

See merge request !188

Fix conflit in TimeSpanDragForceTest

Remove step size limits in analytic propagators

Closes #830

See merge request !192

Align attitude with propagation frame

Closes #586

See merge request !189

Merge changes.xml

Finalized fix for #797

See merge request !193

Sum additions to OrekitMessages

Keep all tests added to CloseEventsAbstractTest,
FieldCloseEventsAbstractTest.

Remove unused imports, fix bug in test where wrong value was used in
assertion.

Also remove Propagator.DEFAULT_LAW. Use IntertialProvider.of(Frame)
instead. See discussion on !189. Part of #586.

Anne-Laure did most of the work to add more informative messages when
root finding for event detection fails. This commit adds a few polishes.
- Added Anne-Laure Lugan as a contributor
- Updated messages to print g values to full precision.
- Made try {} as small as possible since catch() is so broad
- Fix checkstyle warnings
- Add test to check message is formatted correctly

Issue 797 : Improve investigation while encoutering a MathIllegalStateException

See merge request !165

Fix #685 analytic propagator null reset state

Closes #685

See merge request !191