Added convert_dat_to_kwd function that converts a DAT file to an existing KWD file.
Added add_recording_in_kwd function that adds a recording to a single KWD file.
The DatRawDataReader now has a dtype_to argument to specify the data type of the chunks. By default, it is the same as the original data type, so that no conversion is done.
Fixed convert_dtype function and added unit test.