Dynamic popups

[MichaelLangbein]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/dlr-eoc/ukis-frontend-libraries/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)
• [x] The Project is building and passes all tests without errors

PR Type

What kind of change does this PR introduce?

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

• Only the first popup on a raster-layer is being created, whereas all subsequent clicks won't lead to a new popup appearing.
• Dynamic-popup field getAttributes should be optional, but wasn't: Issue Number: 86

What is the new behavior?

Both bugs fixed.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

Is it part of one/more packages and which?

• @dlr-eoc/map-ol
• demo-maps