added validators to PSD class

Author: patrickboateng

geolysis/soil_classifier.py

@@ -426,6 +426,22 @@ def __init__(
         self.sand = sand
         self.size_dist = _SizeDistribution(d_10=d_10, d_30=d_30, d_60=d_60)
 
+    @property
+    def fines(self):
+        return self._fines
+
+    @fines.setter
+    def fines(self, fines: Annotated[float, MustBeNonNegative()]):
+        self._fines = fines
+
+    @property
+    def sand(self):
+        return self._sand
+
+    @sand.setter
+    def sand(self, sand: Annotated[float, MustBeNonNegative()]):
+        self._sand = sand
+
     @property
     def gravel(self):
         """Percentage of gravel in soil sample (%)."""