Convenience: shorter code to build decor.

[thorstenhater]

Add method chaining to decor::{place, paint, set_default}, so that we can now write

auto decor = arb::decor{}
  .paint("soma"_lab, arb::density("hh"))
  .paint("dend"_lab, arb::density("pas"))
  .set_default(arb::axial_resistivity{100}) // [Ω·cm]

to save some redundant typing. Adds the same to Python. Also modernise Python examples
where obvious.

Related to: #1862

[brenthuisman]

One nice task left is to go through the tutorials and correct all the line numbers :)

[schmitts]

I like! Ack to checking the line numbers for the generated docu.

[Helveg]

This is why method chaining never took off in Python 😛 We should pick a format and stick with it though, so let's hash this out :) There's more occurences, but I just picked 2 archetypal ones to propose my solution.

[thorstenhater]

One nice task left is to go through the tutorials and correct all the line numbers :)

I'll address that separatly, since there is more in the pipeline breaking them again.

[brenthuisman]

1. The comments left on examples generalize to the other examples, so as soon as we agree, let's update them all.

2. Then also update line numbers in the tutorials.

[thorstenhater]

This is why method chaining never took off in Python 😛 We should pick a format and stick with it though, so let's hash this out :) There's more occurences, but I just picked 2 archetypal ones to propose my solution.

Yes, I think, I'll add in another layer like this before we are done here (another PR)

d = A.decor(paint=[('(all)', A.density('hh'), ...],
            place=[('(location 0 0.5)', A.spike_detector(-40)), ...],
            ...)

where you could pass anything iterable that yields tuples of locations and paintables/placeables.

EDIT: Checked that out, result is discouraging.

[thorstenhater]

1. The comments left on examples generalize to the other examples, so as soon as we agree, let's update them all.

   2. Then also update line numbers in the tutorials.

Again, the other convenience PR breaks numbers as well. Introducing black will do it again.
I am not going to fix it three times. Let's do it together with black. If we feel uncomfortable with
having an inconsistent state for a day or so, I'll rebase all three PR's together. That is, if we agree
on merging.

[brenthuisman]

Where is this file used?

[thorstenhater]

You hand it a .py in our tutorial style and it will hand you back a list of

.. literalinclude::

with the line numbers between # (N) Section blocks.

[brenthuisman]

Good that you remind me: you can add that or at least one line up top stating what this file can do.

My question was though where it is used. Or is it meant as a helper wrt the tutorial/notebook situation later on?