[BPMApp] Set SPAN of FFT records

[fernandohds564]

Currently, before performing a triggered acquisition we have to set manually the SPAN of all FFT records in order for the plugin to calculate the FFT appropriately. It would be useful if the IOC did that automatically whenever we change the number of SamplesPre and SamplesPost, to avoid operation errors and also simplify the procedure. If the user wants a different value it would still be possible to change the SPAN to the desired value before performing an acquisition.
With this implementation, the value set by the IOC can be thought of as a default value for the SPAN.

[fernandohds564]

Talking offline with @lerwys, he told me that it is better if no default value is set by the IOC. Is that right, @lerwys?