In response to the CII assessment.

Updated list of canonical names and email addresses of contributors

The file `.mailmap`, which enables displaying the canonical name and email addresse of the contributors in the Git log, was outdated. Before its update, 49 different authors appeared in the log, after the update, only 33 authors appear.

For your information, Git can be configured to to use the `.mailmap` file by default in log through to the command below:

```bash
git config --global log.mailmap true
```

See merge request orekit/orekit!4

Readme enhancement

Rewriting of the README file in Markdown lightweight markup language, as suggested in #541.

See merge request orekit/orekit!3

Fixes #544

This reverts commit f8d2a4a2.

Working tests for non-Cartesian orbit propagation are still needed.
Partially fixes #403

In interfaces radiationPressureAcceleration and dragAcceleration, and
all their implementations and their tests.

Partially fixes #514

Fixes #536

Fixes #539

Conflicts:
	src/changes/changes.xml

Fixes issue #538

Fixes #535

Work In Progress, the flattening is not used yet!

This should fix a minor vulnerability CVE-2019-9658, related to default
loading of external entities.