Extend Simple GeoPointField Type to 3d [LUCENE-6480]

[LUCENE-6450 \| https://issues.apache.org/jira/browse/LUCENE-6450] proposes a simple GeoPointField type to lucene core. This field uses 64bit encoding of 2 dimensional points to construct sorted term representations of GeoPoints (aka: GeoHashing).

This feature investigates adding support for encoding 3 dimensional GeoPoints, either by extending GeoPointField to a Geo3DPointField or adding an additional 3d constructor.

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Migrated from [LUCENE-6480](https://issues.apache.org/jira/browse/LUCENE-6480) by Nick Knize (@nknize), updated Feb 01 2016
Attachments: [MortonEncoding3D.java](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6480/MortonEncoding3D.java)
Linked issues:
- #7757

Start by reading the attached MortonEncoding3D.java and linked issue #7757, then compare the proposed approach with Lucene's existing GeoPointField design. Done means the 3D encoding and API direction are decided and the resulting implementation scope is clear.

Written by the indexing model from the issue text.