fix(mtnetwork): add import check

try to import HyPhy before use
in transmission_network.fit_degree_distribution

closes #40

hivclustering/mtnetwork.py

@@ -2496,6 +2496,11 @@ def test_edge_support(self, sequence_records, cycles, adjacency_set, hy_instance
 
     def fit_degree_distribution(self, degree_option=None, hy_instance=None):
         if hy_instance is None:
+            try:
+                import hppy as hy
+            except ImportError:
+                raise ImportError(
+                    "Optional dependencies not found. Please install the 'edgefiltering' extras to use this function: pip install hivclustering[edgefiltering]")
             hy_instance = hy.HyphyInterface()
         script_path = os.path.realpath(__file__)
         hbl_path = os.path.join(os.path.dirname(script_path), "data", "HBL", "DegreeDistributions.bf")