Support dynamic PSF kernel overrides in LensImage

[astroskylee]

This is useful for workflows where the PSF is itself inferred or corrected during optimization / sampling, so the kernel cannot remain fixed at LensImage

Changes

• herculens/LensImage/Numerics/convolution.py

  • PixelKernelConvolution.convolution2d(..., kernel=None)
  • PixelKernelConvolution.re_size_convolve(..., kernel=None)

• herculens/LensImage/Numerics/numerics.py

  • Numerics.re_size_convolve(..., psf_kernel=None)
  • Numerics.render_point_sources(..., psf_kernel=None)
  • dynamic kernel override is supported for regular-grid PIXEL PSF convolution
  • dynamic kernel override is explicitly disallowed when supersampling_convolution=True

• herculens/LensImage/lens_image.py

  • source_surface_brightness(..., psf_kernel=None)
  • lens_surface_brightness(..., psf_kernel=None)
  • point_source_image(..., psf_kernel=None)
  • model(..., psf_kernel=None)