Use pip to maintain dependencies for plugins

[mfitzp]

Currently all dependencies for plugins need to be shipped with Pathomx itself. This limits plugins to using packages that we provide, which shouldn't be the case. We can support custom package install via pip e.g.

import pip
pip.main(['install','scipy','--upgrade'])

We can use this to auto-upgrade packages as plugins depend on them. Define these dependencies in the .pathomx-plugin definition file and auto-run the upgrade-script on startup for all activated plugins (needs to happen before any tools are dragged to the workspace or may error) and any activated after.

We can run the installation code on the kernel to keep the UI free.

[mfitzp]

Some notes

• pip may not working on Windows (for non pure-Python packages)
• need to specify a local <site-packages> folder for the Pathomx application so we can handle all this internally

[aeroaks]

just wondering if conda packaging could provide help with this issue and #11 .