add keys class to interfile module

mu_map/file/interfile.py

+from dataclasses import dataclass
 import os
 from typing import Dict, Tuple
 
@@ -5,6 +6,25 @@ import numpy as np
 
 Interfile = Tuple[Dict[str, str], np.ndarray]
 
+@dataclass
+class _InterfileKeys:
+    placeholder: str = "{_}"
+
+    _dim: str = f"matrix size [{placeholder}]"
+    _spacing: str = f"scaling factor (mm/pixel) [{placeholder}]"
+
+    data_file: str = "name of data file"
+    n_projections: str = "number of projections"
+    bytes_per_pixel: str = "number of bytes per pixel"
+    number_format: str = "number format"
+
+    def dim(self, index: int) -> str:
+        return self._dim.replace(self.placeholder, str(index))
+
+    def spacing(self, index: int) -> str:
+        return self._spacing.replace(self.placeholder, str(index))
+
+InterfileKeys = _InterfileKeys()
 
 """
 Several keys defined in INTERFILE headers.