Implementation of append procedure

New features

• Now you can append two EDF file contents into a single one using the Append(x, y Edf) (*Edf, error) function.
• The EDF struct now has some acessible properties to facilitate the access to common data parameters.

Bug fixes

• Reading and writting is now faster due to a better use of buffering techniques.

Known bugs

• The annotations fields are now correctly updated when appending two EDF files.