Add MDCT Dictionary class with basic functions

MdctDictionary allow to build a MDCT dictionary defined by an array
which contains the sizes of the atoms and a window function.
Methods
- `mdct` operates the MDCT transform on a signal for a given
- `mdctOp` operates the MDCT for all sizes given in the array
Inverse MDCT methods for MdctDictionary

Two new new methods for MdctDictionary :
 - imdct : computes inverse mdct for given signal and frame size.
 - imdctOp : operates inverse mdct on the given MdctDictionary for all
   frame sizes given in the class
+ Add two correction to mdct method :
 - copy arrays in windowing part
 - missing factor in post-twidle
Rename MdctDictionary class in Mdct
Change import numpy as np in mdct.py