Merge duplicated data-structure (QCHackers#293)

Fixes QCHackers#292 by merging duplicated data-structure where possible.

The `BlockDimension` data-structure is harder to merge because it
requires `TemplateSide` and merging that in `tqec.position.Direction3D`
introduces an import cycle.

src/tqec/block/block.py

@@ -14,19 +14,12 @@
 from tqec.exceptions import TQECException
 from tqec.plaquette.library.empty import empty_square_plaquette
 from tqec.plaquette.plaquette import Plaquette, Plaquettes
+from tqec.position import Position3D
 from tqec.templates.constructions.qubit import ComposedTemplateWithSides
 from tqec.templates.scale import LinearFunction, round_or_fail
 from typing_extensions import override
 
 
-@dataclass
-class Position3D:
-    # TODO: merge with data-structures in from #285
-    x: int
-    y: int
-    z: int
-
-
 @dataclass
 class ComputationBlock(ABC):
     """An abstract base class providing the necessary interface to implement a block.

src/tqec/position.py

@@ -1,6 +1,6 @@
 import typing as ty
+from dataclasses import astuple, dataclass
 from enum import Enum
-from dataclasses import dataclass, astuple
 
 from tqec.exceptions import TQECException