sources.jar

An archive for sources can be downloaded
directly from Gitlab, in pages related to
tags or releases.
Uses the standard maven source plugin instead
of rebuilding it with assembly.

To be updated with the key

Add Hipparchus build if necessary

The double hash is used by Velocity as a comment marker.

Conflicts:
	src/test/java/org/orekit/rugged/intersection/BasicScanAlgorithmTest.java

A new method getAlgorithmId() gives the algorithm identifier (DUVENHAGE,
CONSTANT_ELEVATION_OVER_ELLIPSOID, ...)

Fixes #381

Internally, we really use Gradient as the implementation in the library,
and DerivativeStructure in the tests for cross checks.

Fixes #385.