Added draft for batch correction and wrapper functions

Hi all,

This is a draft for the batch correction and batch wrapper functions in issues #23 (closed), #24 (closed), #25 (closed), #26 (closed) and #27 (closed). Instead of individual functions per method, I propose one function which takes a "method" argument, which makes it easy to extend with further batch correction methods in the future.

I didn't test anything at all, so I would be grateful for further development on these functions (therefore also the 'draft' tag)!