Avoid breaking pipeline for unconfigured SonarQube

See merge request orekit/orekit!177

These variables are necessarily set due to initial declarations
at the top of gitlab-ci.yml.

As quality should be checked, a failing stage is introduced to keep
integrator aware.

Used a separate Comparator for sorting integer least square solutions.

Closes #626

See merge request orekit/orekit!176

Display and track coverage global value

See merge request orekit/orekit!175

# Conflicts:
#   .gitlab-ci.yml

The reference value used for the test is updated for the following
reasons:

1/ No reference for the origin of value used in the test
2/ The fixed issue is consistent with IERS 2010 equations

Fix YAML formatting warnings

See merge request orekit/orekit!168

Used the field-specific value of π.

Closes #799

See merge request orekit/orekit!174

Fixes #799

Fixes #821