D3blocks

* Fix for Chord in case input dataframe contains non-unique values

* fix for import sklearn (should have been scikit-learn) Issue 6

* Some troubles with the previous versioning which required releasing a new version.

* fix for import sklearn.

* Removed sklearn to retrieve example datasets.

* Code refactoring across all blocks!
* Removal of `datetime`, `dt_format `and `cmap `as input parameters during initialization of `d3blocks()`
* input parameter `s` becomes `size` across all blocks
* input parameter `c` becomes `color` across all blocks
* Output is consistently stored in `d3.config` `d3.edge_parameters` and `d3.node_parameters`
* Chord edge and node colors can be changed
* Chord edge and node opacity can be changed
* All docstrings are updated with improved examples
* Unit tests are improved
* Bug fixes and code cleaning