Basic Alembic support

All in all, this patch adds an Alembic importer, an Alembic exporter,
and a new CacheFile data block which, for now, wraps around an Alembic
archive. This data block is made available through a new modifier ("Mesh
Sequence Cache") as well as a new constraint ("Transform Cache") to
somewhat properly support respectively geometric and transformation data
streaming from alembic caches.

A more in-depth documentation is to be found on the wiki, as well as a
 guide to compile alembic: https://wiki.blender.org/index.php/
User:Kevindietrich/AlembicBasicIo.

Many thanks to everyone involved in this little project, and huge shout
out to "cgstrive" for the thorough testings with Maya, 3ds Max, Houdini
and Realflow as well as @fjuhec, @jensverwiebe and @jasperge for the
custom builds and compile fixes.

Reviewers: sergey, campbellbarton, mont29

Reviewed By: sergey, campbellbarton, mont29

Differential Revision: https://developer.blender.org/D2060

release/scripts/startup/bl_ui/space_info.py

@@ -158,6 +158,8 @@ class INFO_MT_file_import(Menu):
     def draw(self, context):
         if bpy.app.build_options.collada:
             self.layout.operator("wm.collada_import", text="Collada (Default) (.dae)")
+        if bpy.app.build_options.alembic:
+            self.layout.operator("wm.alembic_import", text="Alembic (.abc)")
 
 
 class INFO_MT_file_export(Menu):
@@ -167,6 +169,8 @@ class INFO_MT_file_export(Menu):
     def draw(self, context):
         if bpy.app.build_options.collada:
             self.layout.operator("wm.collada_export", text="Collada (Default) (.dae)")
+        if bpy.app.build_options.alembic:
+            self.layout.operator("wm.alembic_export", text="Alembic (.abc)")
 
 
 class INFO_MT_file_external_data(Menu):