Add MRN model.

cosmic_dustbox/models/MRN/mrn.py

+from cosmic_dustbox import sdist
+import astropy.units as _u
+
+sdists = {
+    'gra': sdist.PowerLaw(
+        50*_u.angstrom, 0.25*_u.micron, 10**-25.13*_u.cm**2.5, -3.5),
+    'sil': sdist.PowerLaw(
+        50*_u.angstrom, 0.25*_u.micron, 10**-25.11*_u.cm**2.5, -3.5)
+}