Return src_ywidth from source_pos function if using the FWM method instead of Gaussian

[taylorbell57]

Let's do this in a future PR, but I just meant that it would be nice to be able to get a src_ywidth returned out of this function if one was to use the FWM method instead of Gaussian.

Originally posted by @taylorbell57 in https://github.com/kevin218/Eureka/pull/374#discussion_r920620807

[taylorbell57]

A PSF width can also be estimated using a FWM-based method. As an example, see SPCA's centroid_FWM function in Photometry_Aperture.py link where xw and yw are estimates for the PSF width for Spitzer's photometric data

[taylorbell57]

@cpiaulet tagging you here so we remember to do this later since I can't assign you to this issue