not all extra_requirements pip installable

[JuliaSprenger]

What is the problem?
With the new extra_requirements["all"] field users will try to install neo from pypi using pip install neo[all]. However at least one dependency listed can not be installed via this, as stfio is not a PyPI package.

Suggestions for fixing the problem
We should find a way to either specify the sftio dependency in a way that it can be automatically installed or exceptionally exclude it from the list of "all" extra requirements.

[samuelgarcia]

Lets remove the line 'stimfitio': ['stfio'], from setup if it cannot be installed from pypi.
The error will be run time if it cannot be resolve at install time no ?

[samuelgarcia]

@JuliaSprenger : note that in PR #1180 I haven't find a way to handle this all yet.
It is was dynamic (true python code) and the idea to to avoid dynamic for static declaration.