Fixes issue #538

This should fix a minor vulnerability CVE-2019-9658, related to default
loading of external entities.

Fixes #534.

Use the Action enum from Hipparchus as all three enum definitions were the same.
Avoid needing to translate enums.

Fixes #506

Fixes #532

Change from T extends EventDetector to T extends AbstractDetector<T>, which
specifies that the with* methods will return an object of the same type.

Fixes #389

Fixed issue #531

Previously if OrekitException.getMessage() threw an exception it would cause
the message and stack trace to be lost. Now the localization and message
formatting code is inside a try block so the message identifier and stack trace
are not lost.

Conflicts:
	src/changes/changes.xml