200 simplify netzschleuder support

[IngoScholtes]

Implemented new netzschleuder support, which addresses the following issues:

• [x] simplify implementation by downloading CSV data that are parsed based on new pandas interface
• [x] improve robustness of netzschleuder interface
• [x] direct support for temporal graphs based on timestamped tag in netzschleuder
• [x] support for node and graph-level properties
• [x] retrieval of pre-calculated analyses from netzschleuder (adding them as graph-level attributes prefixed with analyses)
• [x] support for edge properties beyond timestamps
• [x] automatic conversion of array-based properties to torch tensors
• [x] add more unit tests