fixes to metadata tab

[sfmig]

Some fixes to the metadata tab

• make columns for 'Events' and 'ROI' not editable:
  • one option to do this: check in metadata_fields.yaml if a column contains a dict. If so, make those not editable.
  • another option: add a parameter for each field saying if it's editable or not in the metadata tab
• do not export a yaml file for a row if 'File' is not specified
  • could be combined with a button to delete empty rows
• review dates formatting: after exporting a row as yaml the format is changed
• add option to generate yaml files from spreadsheet: see Add csv import option #34

Lower priority:

• add button to delete empty rows
• add autocompletion
• add tooltip to header cells based on metadata_fields.yaml
• bug: header scrolls when I scroll in cell with overflow