add convenience append or increment call to buffer chunk

[guruofquality]

• This is a convenient utility to handle the allocation, memcpy, and ptr math.
• Use it in a few places where we were copying, like buffer accumulator

[guruofquality]

https://github.com/pothosware/pothos/commit/dbf75932bc9b95ad04e7684f49e3c1e609fdfc42