Refactor acquisition logic to handle logical variable spectrum.

When using the logical variable mode, we have a frame being captured for
each logical step. For this reason, now we are dealing with a
3-dimensional data.

The logic for finishing one acquisition has also being simplified.

The NDArray allocation has being moved to the end of the loop, when the
frame callback is actually being called.

Co-authored-by: Guilherme Rodrigues de Lima <guilherme.lima@lnls.br>

Author: gustavosr8