[vdif] Added a datetime member function to VDIFFrameHeader.

Calling this returns the datetime of the start of the data frame
which is calculated from the header information.
[vdif_check] Added a flag to do a simple time-check on the VDIF file.

This will print the start and stop time of a VDIF file and exit if given
the -t, --time-check flag. The time-check is printed regardless but with
the flag present it does not do the per-frame checks. This is useful for
getting simple time info from large VDIF files.
[vdif_check] Relegated the packet size logging to debug.