Make dependencies optional (i.e. shapely)

[gplssm]

The openego group uses oemof.db to instantiate a DB connection to OEDB. The installation of packages dependent on oemof.db is sometimes hard due to its dependencies.
Therefore, we propose to make some dependencies optional for example shapely (maybe additional other dependencies). @gnn agreed on this.

I would like to discuss with u @uvchik. Maybe we can have lunch next days.

[uvchik]

In my opinion the only specific requirement left is oemof-solph.

As results is the only module with a functionality in this package it might be okay.

But we should move this dependency to an optional dependency. In most cases people will have solph installed if they want to move solph-results to a database.

This would help to make oemof.db suitable for other use cases.