Adding methods to return the eval buffer and option to start integration from eval buffer

[kishore-nori]

The PR adds functionality in the spirit of QuadGK.jl's quadgk_segbuf method and eval_segbuf keyword argument. The functionality is written similar to that of the equivalent functionality present in QuadGK.jl.

• added hcubature_return_evalbuf along with the count and print variants, to additionally return the final evaluation partition buffer of the adaptive quadrature.
• added eval_buffer keyword argument to optionally pass an existing partition buffer, that shall be used as the initial partition for the adaptive quadrature algorithm to start with.
• added docs for the above functionality.
• added tests to cover the new functionality.

Will look forward to the comments and suggestion for changes and improvements. Thanks a lot for QuadGK.jl and HCubature.jl.