python: Implement phogen encode

Implement phogen.encode() to encode Bhashes to phogen strings. This
commit also adds unit tests for the above function.

python/passgeny/phogen.py

@@ -0,0 +1,19 @@
+from passgeny import bhash, phogen_map
+
+def encode(bh: bhash.Bhash, ph_len: int):
+    """
+    Consume the Bhash and return a string of length ph_len
+
+    This function generates a phonetic representation of the binary data in
+    Bhash.
+    """
+    phogen = ''
+    ngram = '  '
+    for x in range(ph_len):
+        ngram_space = phogen_map.g_phonetic_map[ngram]
+        n = bh.modulo(len(ngram_space))
+        letter = ngram_space[n]
+        ngram = ngram[1:] + letter
+        phogen += letter
+
+    return phogen