Array arithmetic operators and other arithmetic functions

Implemented vector arithmetic operators and relative tests for pele::Array Please let me know whether there is anything that you think can be improved. It would also be nice to implement an array section operator of the type [<lower bound> : <length> : <stride>] so that arrays can be manipulated in a pythonic way at c++ level to do for example things like v[0:3] += v[3:6] or of that sort.

I have also added a fix to the assign function and the assignment operator =, they now check for self-assignment