Discussion on flying focal point

[Zhentao-Liu]

Is your feature request related to a problem? Please describe.
I recently process a CBCT scan with flying focal point, i.e., different focal points for odd/even frames. Actual focal point usually deviates from the ideal one, and such deviation could be described by "geo.offdetector". Although "geo.offdetector" could be input as an array to describe per frame focal point, but it reports bugs when running FDK algorithm. In FDK func of single_pass_algorithms.py, TIGRE currently performs zeropadding and perweighting2 first if "geo.offdetector" exsist, as shown in code. However, these two functions only accept 2-d vector input right now.

Describe the solution you'd like
Simple reimplementation is needed to make zeropadding and perweighting2 function accept array input.

Additional context
BTW, I wanna ask the unit of "geo.offdetector". I think it should be millimeter.