Bugfix RE-API: DROP_ALL_NANS is hardcoded. (#891)
Bugfix RE-API: DROP_ALL_NANS is hardcoded. (#891)

* fix: the parameter DROP_ALL_NANS is no longer hardcoded.
See: https://github.com/nilmtk/nilmtk/issues/890

* refactor: remove unused parameter from the experiment function in API.

* refactor: clean __init__ declarations;
Use .get() instead of double variable declarations.
nilmtk/nilmtk,nilmtk/nilmtk