To be updated with the key

Add Hipparchus build if necessary

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

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.

Fixes #386

Fixes #386