add mechanism for tool discovery

[ltalirz]

Motivation

For this project to grow successfully, I think we want the following to be true:

1. Adding a new tool should be as easy as possible
2. Adding a new tool should not "bloat" the software (download size, startup time, ...)

Thoughts

For 1. for the moment we could simply have one file per tool and autodiscover all @tools that are defined in this folder.
If we feel necessary, we could later even discover tools provided by other packages via Python entry points, but centralizing development in the beginning has advantages (much easier to refactor globally) and can go a long way.

For 2. we need

• some solution to keep dependencies in check. different solutions can work, e.g.

  • use python package extras to allow users to select the dependencies they want (adds complications though)
  • do not install tool-specific dependencies automatically but rather have the tool prompt the user to install them when it's called
  • ...

• Tools must import their specific dependencies only when they are invoked, not at the tool discovery stage.
  This is already relevant now, since it affects load time of the ipython magic