Issue 23 tensor compartment

Created a Tensor class based on BaseCompartment. The Tensor class takes inputs:

1. bvec: applied gradient direction
2. bval: the b-value
3. axial and perpendicular diffusivities

Output:

1. S: signal along the fibre direction

The axial diffusivity is assumed to the along the z-axis ([0,0,1]).

This resolves issue #23 (closed)