Changes:

* Update to Minecraft 1.15.2.
* Added AbstractPart.rotate and mirror, to allow parts in structures to
   mirror and rotate properly.
* Store required parts offset in NBT, rather than absolute
    * This allows them to work correctly when used in a structure.
* Moved a few more logging messages to require the system property
   "-Dlibmultipart.debug=true" to be present.

Bug Fixes:

* [#10] Fixed MultipartUtil.turnIntoMultipart throwing a NullPointerException.
* Fixed multipart blocks crashing when a block receivers multiple ini packets.

