Allow buffering for any tech

[sjpfenninger]

Problem description

Adding the ability to drop in a technology-internal storage component to any technology would permit buffering demand, supply, transmission, or conversion, for example, to implement demand response with a buffered demand tech, or to implement a one-off stock of a resource that can enter the system via a buffered supply tech.

Steps to implement

Generalise the storage constraints currently implemented for supply_plus

[brynpickering]

This is now possible as of #518 where all technologies except demand can store the flow into the model in the same way as a storage technology. It might make the storage technology group redundant!