Add import of Facade class from oemof tabular's facades module

[MaGering]

This PR

• replaces the copy of the oemof.tabular.facades module in oemof.thermal's facades.py with an import of the required class (Facade) and
• deletes obsolete imports

Related: Issue https://github.com/oemof/oemof-thermal/issues/127

[lgtm-com[bot]]

This pull request fixes 1 alert when merging 86a0f3dbe0d1913a710a26e9cb60f9958496599e into 70b6c5fed926d50b8f57d89bf3a2d12073167114 - view on LGTM.com

fixed alerts:

• 1 for __eq__ not overridden when adding attributes

[c-moeller]

Due to incompatibilities of package requirements when installing oemof-tabular together with oemof-solph v0.4.1, I would like to shift this to v0.0.4 If someone gets this running until tomorrow noon, feel free, otherwise let's shift this.