correct the docstring of the grav-tensor components
Merge pull request #95 from birocoles/esfera-kernel

correct the docstring of the grav-tensor components
creates the kernelxx function
there is a docstring problem
correct the docstring problem in function kernelxx
create the functions kernelxy ... kernelzz
prototype function for calculating gzz by using kernelzz
insert if sphere is None: continue in kernel functions
create the function kernelxx for polyprism
solve the doctest problem by changing top of the prism
create the function kernelxy for polyprism
import division from future
modify the doctests for overcoming rounding errors
kernels receive just one sphere
modify tensor functions for using kernel
include the r in docstring to dela with \
first version of tf with kernel functions
substitute the tf for other using the kernels
create the kernel functions for polyprism
substitute the tensor functions for other using the kernels
substitute the tf function for other using the kernels
improve the doctests
include the kernel functions in the description of the gravmag.polyprism