Reformulate interface

[maxfischer2781]

The current interface has some shortcomings:

• pull mode cannot be done externally, making TRE impossible. Since __next__ cannot take any parameters, it must be used inside the chainlet.
• there is an either-or to pull-push, which the syntax does not imply.
• implementing both pull-push duplicates code.
• the simile with generators is not preserved, where __next__() is equivalent to send(None).

Suggested new interface:

• move all handling of values to send(value), which is consistent with the generator interface.
• calling __next__() is equivalent to send(None).
• implement explicit push(value) and pull(value) which use send to process their input.

[maxfischer2781]

Done with 46b96ee1603f0bb5581daccb5b58efc17e9e4925/v0.10.0. Chainlets must define chainlet_send for handling values.