Fix asterisk notation for mpes reader

[domna]

Axis names from xarray should be collected properly.
We use an asterisk for this to create key-value pairs:

"/ENTRY[entry]/data": {
    "@axes": "@data:dims",
    "AXISNAME_indices[@*_indices]": "@data:*.index",
    "@signal": "data",
    "data": "@data:data",
    "data/@units": "counts",
    "AXISNAME[*]": "@data:*.data",
    "AXISNAME[*]/@units": "@data:*.unit",
    "energy/@type": "binding"
  }

, which creates, e.g., AXISNAME[energy] = @data:energy.data for all names in the data array.
We however, would like to be able to rename the key manually or enter a different value for the key. So if either the value or the key are present in the file the automatism should not write this particular entry. This manual overwriting does not work properly and needs to be fixed.