Bracket access to solute storage

[philippkraft]

The current access to solute (here X) storages of water storages (here WS) is super ugly:

WS.Solute(X).state

This issue proposes a nicer access like:

WS[X].state

Additionally we can move the conc method from water storage to a property of the solute storage

WS[X].conc

[philippkraft]

@smultsch: What do you think?

[smultsch]

For sure, the function call "WS[X].conc" is a really clean solution. But the user needs some foreknowledge, i.e. that the water storage holds a list with solutes.

[philippkraft]

The documentation of the tracer use is not so great now anyways. I guess we can simply write a tutorial about that use.

[philippkraft]

Fixed #29. Usage see test/solute_access.py and demo/adsorption.py. Old usage is still working. Other demos get translated to new usage on the go.