several big changes in PyS_SDSScompspec

- included fitsio python to read the FITS files, since it is much faster than
  astropy. Same name as before with _fitsio at the end
- in order to use the speed, changed the for loop over the files in the main
  function. Instead of running over all files at once, we now read a buffer of
  files, calculate their coordinates and the dust map values and only then
  calculate the rest
  Therefore higher memory usage, but faster.
- new coordinate_arrays class, which stores arrays for ra, dec, l and b, because
  access of individual SkyCoord transformations is very slow. Use the fact that
  one can give arrays to SkyCoord and transform those.
- changed calculation of E(B-V) such that we do this on whole arrays as well,
  also increases speed by a lot
- implemented dust corrections properly. taken dust_extinctions.c and written
  Cython file, which is given flux, flux_error and wavelength arrays.
  needs to be compiled before usage:
  python setup.py build_ext --inplace
  then imported by
  from dust_extinction_array import dust_extinction_array
       - if that fails, we ask if compilation should be tried, if compilation
         not wanted, use a python implementation instead
	 dust_extinction_python.py
	 is slower, haven't tested by how much
- Gives the same results compared to C dust correction up to about 10^(-6)