Should we allow positive index values for historical range access?

[robambalu]

At the moment historical range access (https://github.com/Point72/csp/wiki/Historical-Buffers#historical-range-access) only letes you index from the last value backwards ( 0 being last value, -1 previous tick, etc ... )
There is no convenient way to get the first tick in the buffer though.
One idea is to allow index values > 0 to represent values from the "start" or back of the buffer. So 1 would be the first tick in the buffer, 2 second, etc...
Would be convenient to have this, though I'm open to discussion on the API as having positive index values can be confusing / error prone.
We would also need to consider this in this single value_at case as well as range access