Remove use of scenario field in config.json

[dhblum]

Currently, there are default scenario fields defined in each test case's config.json, which are compiled into the FMU. However, all the configurations set electricity_price to constant and time_period to None. This can be done in testcase.py by default for all test cases. Then, the field can be removed from config.json and when new scenario fields are added (e.g. weather forecast uncertainty in #135 and likely more in future), all test case FMUs don't need to be recompiled with updated config.json.

This issue can be addressed by edits in testcase.py and the config.json files in each test case directory. However, test case FMUs do not need to be recompiled in my opinion to complete this issue. They can be updated over time as-needed by model or other structural changes.