BF: #427 Sort of None echotime was not working. Replaced with nan

Yaroslav adjusted to import nan at the module level and straight
from math module.

Author: Joerg Stadler

heudiconv/convert.py

@@ -2,6 +2,7 @@
 import os
 import os.path as op
 import logging
+from math import nan
 import shutil
 import sys
 
@@ -530,7 +531,7 @@ def save_converted_files(res, item_dicoms, bids_options, outtype, prefix, outnam
 
         # Check for varying echo times
         echo_times = sorted(list(set(
-            load_json(b).get('EchoTime', None)
+            load_json(b).get('EchoTime', nan)
             for b in bids_files
             if b
         )))