Cannot download CSV data for waveform

[djlauk]

What I did
Steps to reproduce the behavior:

1. Go to https://data-ui.psi.ch/preselect?c1=sf-databuffer%2FSARMA01-DBCM030-SPEC%3ASPECTRUM&startTime=2022-03-15T15:52:12.005Z&endTime=2022-03-15T15:52:26.005Z&queryExpansion=0&tooltipEnabled=1&plotVariation=separate-axes
2. Click on button CSV download
3. Click on button download

What I observed

• Message 'Your download has been started in the background. Please wait.' in a snack bar pop-up.
• No download ever happens.
• Error message in JS console 'Error: internal error: not a scalar channel: sf-databuffer/SARMA01-DBCM030-SPEC:SPECTRUM'

What I expected to see instead

• Download of CSV data finishes after some waiting.

Screenshots
If applicable, add screenshots to help explain your problem.

n/a

Computer / Environment (please complete the following information):

• OS and version: [e.g. Windows 10, RHEL 7.1, macOS 10.15.2]
• Browser: [e.g. chrome, edge, firefox, internet explorer, safari]
• Browser version: [e.g. 22]

n/a

Additional context
Add any other context about the problem here.

The channel 'sf-databuffer/SARMA01-DBCM030-SPEC:SPECTRUM' is a waveform channel.

[djlauk]

After discussing this we will implement the following:

• Skip waveform channels in the CSV download
• Add a note in the CSV dialog if there are waveform channels selected, that these are being skipped
• Disable the download button, if all channels are waveforms
• Add waveforms to the notes in the how-to
• Add waveforms and CSV to the FAQs