[stbtel][converter] Swithing to SERAFIND if necessary when converting to SERAFIN

Handling automatic detection of double precision when writting a serafin file. This was coded for stbtel converter and run_telfile.py alter.

The condition for double precision is that at least on segment of the mesh has a length lower than 1e-6.

Test cases where added for both as well as an option to disable the detection.

It should be tested on a more standard mesh before merge.