Add option to disable SM BG interpolation

[jusack]

For some measurements (e.g. hybrid measurements in an MPS) it does not make sense to assume that the BG measurements are acquired between the foreground measurements.

We should add an option to disable the BG interpolation, either by keyword argument or maybe we can extract the information from the MDF

MPIFiles.jl/src/MDFCommon.jl

Line 62 in 206b708

if bgCorrection && length(measBGFrameIdx(f)) > 0 # this assumes equidistant bg frames