fix circular import with trace_shifts

[segasai]

When trying to import desispec.trace_shifts
I get the circular import

File ~/pyenv310/lib/python3.10/site-packages/desispec/image_model.py:25
     23 from desispec.qproc.qsky import qproc_sky_subtraction
     24 from desispec.qproc.qfiberflat import qproc_apply_fiberflat
---> 25 from desispec.trace_shifts import compute_dx_from_cross_dispersion_profiles
     27 @numba.jit
     28 def numba_proj(image,x,sigma,flux) :
     29     '''
     30     Add a spectrum to a model of the pixel values in a CCD image assuming a Gaussian cross-dispersion profile.
     31 
   (...)
     36        flux : 1D numpy array of size image.shape[0], quantity to project (for design usage: electrons per pixel) for each CCD row
     37     '''

ImportError: cannot import name 'compute_dx_from_cross_dispersion_profiles' from partially initialized module 'desispec.trace_shifts' (most likely due to a circular import) (/home/koposov/pyenv310/lib/python3.10/site-packages/desispec/trace_shifts.py)

The patch fixes that

[sbailey]

For the record, desispec.scripts.trace_shifts accidentally avoided this circular import with from desispec.io import read_image before from desispec.trace_shifts import ..., which is why we weren't previously bitten by this.

But I agree that desispec.trace_shifts / desispec.image_model has a circular import and that this PR fixes it. Thanks.