This helper function is needed on some WCS services from which an mht
file is downloaded rather than a tif file; returns the path to the tif
Details
Need three ways to read in the mht file to get the tif file out.
read_lines() cannot read all lines due to embedded nulls.
Therefore, also read_lines_raw() needed for positioning of tif part in
file.
write_lines() does not work correctly on lines_raw[start:end]
possibly a bug or edge case in write_lines()
Therefore, also read_file_raw() needed to extract from the raw vector