load sorted files

[rmojgani]

This ```sorted''

py2d/test.py

Line 102 in 1baad8e

files = sorted(os.listdir(directory), key=lambda x: int(x.split('.')[0]))

does not correctly distinguish order of and sorts as

 a1.mat , a.10.mat, a2.mat

instead use ``natsorted'' that sorts

 a1.mat ,  a2.mat, a.10.mat

The additional dependency can be discussed