Add ToDO file

ToDO

+Improvemnents
+1) Periodic boundary conditions for cubic box
+2) 
+
+
+
+Bugs
+1) Spherical -- check the values of multipole and local coefficients.
+   use :  utestSphericalDirectDebug.cpp 
+2) 
+
+
+ToDO
+1) Change the algorithm for periodic boundary conditions. (Ewald approach)
+2) Non cubic box.
+3) Compute the virial for MD simulations
+4) Hybrid parallelism for the tree build