Flexible start end times

Description

Previously the application required at least one scenario to be present, otherwise no start and end dates could be set. This now has been changed. The start and end dates are now the minimum and maximum of simulation and case data.

Additionally the "Simulation Start Date" has been renamed to "Reference Day". If a simulation is present it will represent the first day of the simulation, otherwise it will represent the first day, where case data is available.

Related Issues

Resolves #300

Checklist

I, the author of this PR checked the following requirements for good software quality:

[x] The code is properly formatted (I ran the formatter)
[x] The code is written with our software quality standards (I ran the linter)
[x] The code is written using our code style
[x] Extensive in source documentation has been added
[ ] ~~Unit and/or integration tests have been added~~
[x] All texts have been internationalized with at least the following languages:
- [x] English
- [x] German
[x] I tried addressing all new accessibility problems displayed in the console and documented if they can't be fixed
[x] I attached performance measurements to prevent performance degradation
[x] I added the changes to the next release section of the changelog

I, the reviewer checked the following things:

[x] I ran the software once and tried all new and related functionality to this PR
[x] I looked at all new and changed lines of code and commented on possible problems
[x] I read the added documentation and checked if it is understandable and clear
[x] ~~I checked the added tests for completeness~~
[x] I checked the internationalized strings for spelling errors
[x] I checked the performance metrics for problems or unexplained degradation
[x] I checked that the changes are noted in the changelog

DLR-SC / ESID