Fix #769, fix #763, fix #756

Changes proposed in this pull request:

• Add -d option to mvs_report command to use hotreload of dash app for devs
• Fix issue ([Bug] Report empty table #763): Avoid displaying a energy sector demand table in report if it is empty
• Fix issue (mvs_report terminates before printing a report #769): Fix argument parsing and error messages of mvs_report command
• Fix issue (Problems when trying to simulate without internet connection #756): Avoid crashing report generation when internet not available
• Update RTD instructions for the mvs_tool command

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code (example docstring)
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code (pytests, assertion debug messages)
• Update the CHANGELOG.md
• Apply black (black . --exclude docs/)
• Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}