Fix to allow for Pipeline object as estimator

This release fixes a bug that occurred when providing a Pipeline object as the estimator when using cross_val_fit. Now Pipeline objects can be provided as the estimator.

added cloning of transformer

Cloning of transformer during _cross_val_fit ensures no leakage between splits.
Edited example in the examples folder after bug fixes.

cloning estimator to fit

Fixed the following issues:
#15
#13

These pertain to the fact that there was leakage in the estimator between splits. Now this is solved as the package clones the estimator before fitting to each split.

prediction bug fix

Fixed issues regarding the prediction function, namely #7.

initial panelsplit release

Initial release of panelsplit.