Add option to set a fixed dithering amount

[zhouconghao]

This enables testing the pixel interpolation effect with a fixed offset between the SE WCS and coadd WCS. Let me know if I should come up with a test or change anything. Thanks!

[zhouconghao]

Somehow the test got an error in conda install. Pasted here

Preparing transaction: ...working... done
Verifying transaction: ...working... done
Executing transaction: ...working... done
warning  libmamba Could not parse mod/etag header
warning  libmamba Could not parse mod/etag header
warning  libmamba Could not parse mod/etag header
warning  libmamba Could not parse mod/etag header
warning  libmamba Could not parse mod/etag header
warning  libmamba Could not parse mod/etag header
Download error (28) Timeout was reached [https://conda.anaconda.org/conda-forge/linux-64/htcondor-utils-23.0.3-h841b71b_0.conda]
Operation timed out after 9053 milliseconds with 0 bytes received
error    libmamba Error when extracting package: std::bad_alloc
terminate called after throwing an instance of 'std::bad_alloc'
  what():  std::bad_alloc
/home/runner/work/_temp/677bb002-520b-44d9-8240-544f958b1106.sh: line 9:  1999 Aborted                 (core dumped) mamba install -q --file requirements.txt
Error: Process completed with exit code 134.

will try later.

[beckermr]

In addition to the comments below, I think we should change the keyword argument to dither_size.