Custom check

[ChristopherMancuso]

@RemyLau For #176 it was reading just the pre-stored networks, and I changed it to read anything in file_loc that has a node order

For #175 if a user had a custom network selected and auto_download on it raises an error, which I think is fine. I just tried to clear up that it is just a problem with trying to download the files.

[RemyLau]

@ChristopherMancuso I have proposed a new solution for fixing #175 (see 737a4cf). Instead of prompting error inside the private _make_download_options_list function, I think prompting warning messages during the initialization of the geneplexus object is better for the following reasons

1. It does not through error
2. It is more localized, i.e., it's easier for the user to find the auto_download option when the error comes from geneplexus.__init__ rather than _make_download_options_list.

[ChristopherMancuso]

@RemyLau That is a much better way to fix #175. I think it all seems good now.