General relativity

I add some functions for:
(1) Tensor calculations in the component form, includes (a) tensor product, (b) partial derivative which gives some higher ranked tensor, and (c) Einstein summation for a tensor with dummy indices.
(2) General Relativity related functions to calculate Riemann curvature tensor, Ricci tensor, and Ricci scalar.

I include a test file which shows that the calculation for Ricci scalar on the surface of a sphere, and for Schwarzschild metric are correct.