Fix spectral index error bars

If more than two frequencies are used, the error bars were wrong for
alpha2-3 and alpha3-4. This update fixes this.

Fixed grid size

- Forced spinteractive to use a grid_size of ~200. This was implemented
in a previous version, but got reverted with the version that fixed the
reduced chi^2
- Spinnaker now uses an internal grid_size that is double the input
grid_size. This allows the convolution with the effective beam to work,
so that the intensities do not decrease towards the edge of the grid

Delete NE_L_nt_N.dat

Delete NE_L_nt_N.dat

Add spinteractive folder

I added a spinteractive folder that contains example data of NGC 5775.
There is also my chi_NE.py script to calculate the reduced chi^2.

Add GPL license.