Add FITS header check as a user option

[JulienPeloton]

The PR #55 fixed a bug with the header check (the connector was checking all FITS header before starting the job. Good idea until you have +10,000 files....). The fix is however temporary, as you can see below:

// Check that all the files have the same Schema
// in order to perform the union. Return the HDU type.
// NOTE: This operation is very long for hundreds of files!
// NOTE: Limit that to the first 10 files.
// NOTE: Need to be fixed!
val implemented = if (listOfFitsFiles.size < 10) {
  checkSchemaAndReturnType(listOfFitsFiles)
} else{
  checkSchemaAndReturnType(listOfFitsFiles.slice(0, 10))
}

The idea would be then to add an option to the DataFrameReader (e.g. checkHeader), false by default, that would trigger the header checker.