Litemapy

* Support for nbtlib 2.0
* Drop support for python 3.7

* Bugfix: Save timestamp as milliseconds instead of seconds

* Made BlockState immutable.
* BlockState properties should now be accessed directly from the blockstate instead of from its no longer visible properties field (e.g. `blockstate["propertyname"]` instead of `blockstate.properties["propertyname"]`).
* Correctly handle multiple regions in a single schematic. Schematic's enclosing sizes are no longer supplied by the user but rather calculated from the schematic's regions.
* String representation of BlockStates.
* Helper methods to iterate over region coordinates and get a region extrem coordinates in its schematic.
* Helper method to create a schematic from a single region.
* Use numpy for storage.

* Bug fix by SebbyFur : Properly initialize block state properties
* Refactored tests

* Modification time is now updated when writing a file
* Refactored constants into a separate file
* Added a software metadata to help identify schematics created with litemapy and the version used
* Added a DiscriminatoryDictionnary class to store regions in schematics while making sure they fit in
* Regions are now stored in a read only discriminatory dictionnary of the schematic, using their names as key
* Trying to add a region that does not fit into a schematic raises an error
* Region's names are not stored in region anymore
* Region's positions and sizes are now read only
* Region's palettes are now private
* Schematic's size is now read only

* Refactored package