Loading of voc.txt takes too long

[paul-michalik]

Loading of the BoW vocabulary must be accelerated. Investigate possibilities to accomplish this.

[shanmukhananda]

Current numbers on my PC (i7, 4cores-8threads, 2.6GHz, 16GB RAM)
Windows Release build takes 40-42 seconds to load orb-voc, (8 cores, 16gb RAM)
Ubuntu Release build takes 7-9 seconds to load orb-voc (4 cores, 8gb RAM)

Ideas on improving

• Use binary vocabulary instead of txt file.
• In windows we have to remove /RTC and try /O2. (VS optimization flags). Linux optimization flags are already in place

[shanmukhananda]

Tried with binary vocabulary referring this link
Vocabulary loading time now reduced to 350-450ms both in linux and windows.

[shanmukhananda]

changes merged cf74e99