Fix precission errors in the database

[jdavid]

This is a follow up to issue #9

We've (pending verification):

• Fixed the rounding issue for new frames, from the mote to the server
• Fixed the rounding issue in the Pi/Server, for motes still running the old sketch

We still have to:

• Fix the rounding issue for frames already stored in the server

Do this only when the issue #9 has been verified.