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>

src/bmtf_orbit_processor.h

@@ -16,7 +16,8 @@ class BmtfOrbitProcessor : public OrbitProcessor {
   }
 
   inline uint32_t GetOrbitSizeInBytes(const FillOrbitMetadata &meta) const override {
-    return GetPacketSize() * meta.filled_bxs;
+    // Each BX requires 3 words (12 bytes); each stub takes 8 bytes
+    return 12 * meta.filled_bxs + 8 * meta.counts;
   }
 };