Change FFT memory Error to Warning?

[sidneymau]

Currently, GalSim will throw an error if the requested FFT is too large (cf. https://github.com/GalSim-developers/GalSim/blob/4c04a1ea4aa4c652c25019c0c7b5881f7571035f/galsim/gsobject.py#L1970-L1971).

The default value for this is quite low (something like 2GB) and this often results in many jobs crashing and hanging. Every time I configure this to be larger, I still end up finding some object that needs more memory resulting in another crash. It would be helpful for this to be changed into a warning rather than an error.