Replace GeometryStream with GeometryMap.

[GerardasB]

This PR suggests to change GeometryStream property of GeometryDetailRecord element to GeometryMap.
The main reason for the change is - possibility of huge number of elements and resulting performance issues.

There are some major complications when using GeometryMap, instead of i.e. using relationships to group related geometries.

1. Geometry of GeometryMap is not an Element. I.e. additional properties need to be serialized.
2. GeometryMap entry id's are not unique in the iModel - i.e. to reference a geometry in a GeometryMap one needs to provide entry id of GeometryMap AND element id of GeometryDetailRecord. Also managing such relationships.

Suggested FlatBuffer schema for GeometryMap:

table GeometryMap {
  entries: [GeometryMapEntry];
}

table GeometryMapEntry {
  id: int;
  entries: [ElementGeometryDataEntry];
}

table ElementGeometryDataEntry {
  opcode: int;
  data: [ubyte];
}

[caseymullen]

Has Brien's code to work with GeometryDetailRecords also been updated to work with GeometryMap instead of GeometryStream?

[GerardasB]

@bbastings Looks like there won't be any issues to update the code if/when we decide to merge that branch.

[bbastings]

Agree.

[caseymullen]

Need a .remarks.md file documenting the format or linking to its documentation. Should also link to docs for the API for working with this stuff.

[GerardasB]

Added the remarks, the API is not published to public registry yet.