Prevent loading of IWU-dataset from slowing down the calculation

[lensum]

Is your feature request related to a problem? Please describe.
When reading the TEASER examples, I found a note saying that when a new Project-instance is created, the keyword load_data=True leads to a lot of IO and thus can slow down the calculation if performed multiple times.
In the current version of the districtgenerator, in datahandler.py line 202, the Project instance is created with load_data=True and in line 214 new data is loaded by prj.add_residential(method='tabula_de', ...). I assume, that this slows down the calculation, though I haven't tested it on significance.

Describe the solution you'd like
Line 202 could be changed to read prj = Project(load_data=False).

Additional context
As I've said, I haven't tested the change yet. The effect on calculation time could be insubstantial.