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.

Previously the test test of the compiler plugin was not compiled. Now it
is compiled and run.

Previously TSF.getGMST(conventions, simpleEop) would always return the
same value regardless of the parameters. Pull up the implementations of
getGMST and getUT1 from PreloadedTimeScales to AbstractTimeScales and
used them in LazyLoadedTimeScales. Added test cases for
LazyLoadedTimeScales.

Fixes #627