Adjust Open File for varying network modes (GRN/PPI)

[dondi]

The Open File functionality needs to support PPIs more “natively” than they do now. Specifically:

• only .xlsx and .sif should be allowed for PPI, not .graphml
• check .sif format for PPI

In addition, instead of having the Network Mode lock in which files can be opened, would it be possible for the open-file command itself to determine the Network Mode based on the selected file? e.g., if the user picks a GraphML file, then we switch to GRN network mode for sure. Also if opening a demo, we can switch the Network Mode automatically at that time

For .sif files, an edge label of pp indicates a PPI. Thus, “pre-reading” a .sif can also set Network Mode upon opening