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 orekit/orekit!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 orekit/orekit!165

Fix #685 analytic propagator null reset state

Closes #685

See merge request orekit/orekit!191

Fix #618 ClasspathCrawler match file name only

Closes #618

See merge request orekit/orekit!190

And also the related FieldPropagator.DEFAULT_LAW and
IntertialProvider.EME2000_ALIGNED. Left a couple usages in the tests
that were necesscessary for proper class loading and for testing the
deprecated values. #586