Get the time interval between the frames of an IMX/IM7 file
DT = getframedt(FILENAME) explores the 'AcqTimeSeries' attributes
of a DaVis multiframe image matching FILENAME (format IMX or IM7) and
computes the time intervals DT between the first frame and the
following ones (in ms).
If the file FILENAME contains N frames, then DT is an array of size N-1
(N=2 for classical PIV applications, so DT is a single number).
If N=1 (single frame image), then DT is 0.