ENH: add time variable density to Tank Fluids.

rocketpy/motors/fluid.py

@@ -1,3 +1,5 @@
+import numpy as np
+
 from dataclasses import dataclass
 from typing import Union
 
@@ -47,6 +49,10 @@ def __post_init__(self):
             if self.density < 0:
                 raise ValueError("The density must be a positive number.")
 
+        elif isinstance(self.density, Function):
+            if (self.density < 0).any():
+                raise ValueError("The density must be a positive number.")
+
         self.density = Function(
             self.density,
             interpolation="linear",

rocketpy/motors/tank.py

@@ -86,7 +86,7 @@ def __init__(self, name, geometry, flux_time, liquid, gas, discretize=100):
             Geometry of the tank.
         flux_time : float, tuple of float
             Tank flux time in seconds. Time interval that the fluid flux is
-            being analyzed.  If a float is given, the flux time is assumed to
+            being analyzed. If a float is given, the flux time is assumed to
             be between 0 and the given float, in seconds. If a tuple of float
             is given, the flux time is assumed to be between the first and
             second elements of the tuple.