MAD-X Reader: Avoid Scipy

Currently, we do not need `scipy` outside of examples. This is
neat, because it is a sometimes hard to install dependency.

This updates `madx_to_impactx.py` to avoid requiring scipy for
a few constants.

In the future, if we have a hard dependency on `scipy`, we should
add it in `requirements.txt` of the project root.

src/python/impactx/madx_to_impactx.py

@@ -8,13 +8,28 @@
 
 import warnings
 
-import scipy.constants as sc
-
 from impactx import RefPart, elements
 
 from .MADXParser import MADXParser
 
 
+class sc:
+    """
+    This class is used in lieu of scipy.constants
+    to avoid a direct dependency on it.
+    At the time of writing, this file was the only
+    one requiring scipy in the ImpactX source outside
+    of examples.
+    """
+
+    c = 299792458.0
+    electron_volt = 1.602176634e-19
+    physical_constants = {"electron-muon mass ratio": (0.00483633169, "", 1.1e-10)}
+    m_e = 9.1093837015e-31
+    m_p = 1.67262192369e-27
+    m_u = 1.6605390666e-27
+
+
 def lattice(parsed_beamline, nslice=1):
     """
     Function that converts a list of elements in the MADXParser format into ImpactX format