into develop

Remove production of sources.zip

See merge request !45

Compile regex once and reuse it instead of recompiling it for each line
parsed. Resulted in a speedup of 1.2 for an example OEM loading test
case.

Fixes issue #640

Doc: declare process for site

See merge request orekit/orekit!44

Fix download links

See merge request orekit/orekit!43

An archive for sources can be downloaded
directly from Gitlab, in pages related to
tags or releases.

Use dedicated image for build

See merge request orekit/orekit!42

New line property should now be valid for linux, windows and mac os
users.

Fixes issue #560.

Fixes #639

This image embed both maven and graphviz.

The effective real download is on Maven Central.
The projects's Nexus is used for releasing
process.

Remove a point discontinuity and canceled terms analytically to avoid
catestrophic cancellation. Added a test case, thanks to qmor for
reporting the issue.

Fixes #639

This should now avoid numerical issues when computing the discriminant.

Fixes issue #639.