bugfix for CSV reading: default encoding now utf-8 as this includes ascii

Filereader utils would try to determine the encoding using the first 10,000 characters when importing a file.
Should a CSV contain a ascii up till the 10k threshold but still contain utf-8 characters, the file reader would raise encoding error.

However as utf-8 is backward compatible with ascii, it is more robust to default to utf-8 instead of ascii.