Added first datas

datas is a module containing Data Structure classes. This initial Class has a Symmetric Dictionary, intended to simulate a symmetric matrix with a default value assigned to the diagonal.
That is:
>> x = sdict()
>> x[1,2]=3
>> x[2,1]
>> 3
>> x[0,0]
>> 1000000.0
>> x[1,1]
>> 1000000.0