Populate registries after you do load a pickle file

[ambujpawar]

• Add new to the SimpleImputer
• This enables us to populate registries as pickle.loads does not call init of a class
• Modify test to check just the name of metrics instead of complete metrics

Closes #10

NOTE: This code only solves issues with Imputer. For failing tests while preprocessing please check #23

[MBrouns]

Looks good! If possible I'd like the changes for both this and #23 in the same PR. Also, I think the __init__ implementation is now unnecessary. Could you check whether it can be removed?

[ambujpawar]

Yes, you are right. dunder init is no longer needed.

Changelogs:

• Removed init
• Pulled the changes from Add __new__ to the OneHotEncoder class #23 into this PR(credits: @a-debruijn )