fix: bmtf: Change filled orbit size

Tweaked BMTF's GetOrbitSizeInBytes() to reserve 8 bytes for each
existing stub, plus 3 words for each filled BX.

Fixes issue #64.

Co-authored-by: Giovanna Lazzari Miotto <giovanna.lazzari.miotto@cern.ch>
Co-authored-by: Rocco Ardino <rocco.ardino@cern.ch>