Cloning functionality + Midje unit tests

[ashimapanjwani]

Added prevent-clone? as an optional parameter to the following functions:

• dataset
• add-column(s)
• map-columns
• update-columns

Other functions have been skipped since they don't seem to be computationally intensive. In case the user wants to use the cloning functionality in any other case, they can use the clone-columns function to do so.

[genmeblog]

Let's discuss a little bit more what should be the default bahaviour: clonning or not clonning (that's the question :) ). Raised by @behrica (cc. @daslu ) https://clojurians.zulipchat.com/#narrow/stream/265544-sci-fu/topic/sci-fu.20session.2011.2E2