Added numpy() method

[alexander-g]

kp.Tensor.data() returns a list of floats which is neither convenient nor efficient for further processing. I have added the .numpy() method which directly returns a numpy array.

[axsaucedo]

Very cool @alexander-g, good addition! Would you be able to add a test in the python test in the test folder? Mainly to make sure it works as expected before doing the releases. You will have to add numpy to the reuirements-dev.txt. If you get a chance to add further docuentation that is a plus, but it gets much more fiddly on how to build the docs so we can do that separate to this PR

[axsaucedo]

Fixes #104

[alexander-g]

Added tests.
Docs have to wait until I have figured out how to build them

[axsaucedo]

Looks good @alexander-g, thanks for the contribution