Make Limit_cycle module compatible with the linear response module

[oameye]

• Check on a toy model if gives the right results

[jkosata]

Is it ok if I pick this up or do you have some more local stuff to push? I was going to write a short doc page for the limit cycles to submit with our revised paper

[oameye]

@jkosata Yes ofcourse! Feel free to push this forward. Maybe, merge master with this branch first, before you continue.

[jkosata]

Rebased to master and did some minor touches, more coming

[jkosata]

I've brushed up the LimitCycles module so that our new paper is reproducible, removed a small bug, added an example doc.

The output of plot_linear_response for limit cycles looks reasonable and we've used it successfully before, but I can't think of a simple test.