default dtype raises ValueError when initializing a Network

[la-bell]

Potentially similar to issue #88 but seems like a distinct issue.

The __init__ method for the Network class creates a node_idx attribute as a pd.Series whose dtype is specified as "int". This yields a Series with int64 as its dtype. I don't know if this is due to a version update in pandas or numpy or something else, but it breaks the call to cyaccess, which expects node_idx to be long but is now getting long long. I tried coercing all inputs to np.int32, but it did not resolve the issue. The problem seems to be the dtype="int" in line 86 of network.py. If I change it to dtype=np.int32 my code executes. This seems like a more explicit specification in any case, so it may be preferable to simply having "int".