Update the default value of PS_CONVERT and the begin module

[seisman]

In #5583, some modifiers of -A options are moved to the -I option. Now, -I should be used to crop the image.

The default value of PS_CONVERT is A, but now it should be I.

The "begin" module also need to be updated.

[PaulWessel]

Hm, no. -A is still the option that crops the image. -I adjusts the BB further (from original or cropped size), and N colors it. So PS_CONVERT default will remain A.

[seisman]

I think I misunderstood these two options.