Python implementation of folding, with relatively simple fortran subroutine

To compile and compare (for quicker comparison, divide `nt` by 128 in both `test_read_gmrt.py` and `read_gmrt_ueli.f90`)
```
cd scintellometry/folding
f2py --fcompiler=gfortran -m read_gmrt -c fortran/read_gmrt.f90
python test_read_gmrt.py
cd fortran
gfortran -O3 read_gmrt_ueli.f90
./a.out
diff dynspec1957+20.bin ..
```