switching to a new city during load of another can cause race condition problems

[robstoll]

Describe the bug
If one triggers two loadCityData then the last wins which can cause the situation, that the wrong fountains are emitted as event

To Reproduce
Steps to reproduce the behavior:

1.goto https://beta.water-fountains.org/ch-zh
2. Switch to Geneva before the fountains are loaded
3. Depending on which http call finishes first, the wrong data is displayed (e.g. zurich instead of geneva) or no fountains are displayed (geneva is shown but the fountain dots of zurich are rendered on the map)

Expected behavior
pipe the calls so that the first is canceled in case we change the fountain before

Screenshots

Additional context
Add any other context about the problem here.