Improved buffer / filler handling

[cgevans]

At the moment, buffer is treated as a special addition to mixes, not as a component.

I think it would make more sense to have a buffer / fill action, which would fill any remaining quantity with the component in that action. An experiment could have a default fill action to add to mixes.

This would allow handling of situations where the filling component changes: for example, if someone is storing their DNA in water without buffer, and then adds buffer at some point in mixing, then wants to preserve that buffer in later stages, even if potentially adding in some other things that are in water without buffer.

[dave-doty]

for example, if someone is storing their DNA in water without buffer, and then adds buffer at some point in mixing

The way I handle this currently is to set buffer_name to "water", and then my 10x buffer is just another component.