Replace unyt.amu with Unit("amu") to perserve mass values from mB…

…uild (#759)

* Fix amu unit usage from Unyt

* remove commented out line

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gmso/external/convert_mbuild.py

@@ -5,6 +5,7 @@
 import numpy as np
 import unyt as u
 from boltons.setutils import IndexedSet
+from unyt import Unit
 
 from gmso.core.atom import Atom
 from gmso.core.bond import Bond
@@ -271,7 +272,7 @@ def _parse_site(site_map, particle, search_method, infer_element=False):
         ele = search_method(particle.name) if infer_element else None
 
     charge = particle.charge * u.elementary_charge if particle.charge else None
-    mass = particle.mass * u.amu if particle.mass else None
+    mass = particle.mass * Unit("amu") if particle.mass else None
 
     site = Atom(
         name=particle.name,