Replace a few occurrences of `atan(Y/X)` with `atan2(Y, X)`

[WarrenWeckesser]

A potential bit of code maintenance:

I noticed a few occurrences of atan(Y/X) in the xsf code. Generally these can be written atan2(Y, X), which avoid a potential division by 0 and gives results from (-pi, pi) instead of (-pi/2, pi/2). Here's what a search with a regex finds:

$ rg "atan\([^)]*/.*\)"
specfun/specfun.h
706:                        phi = atan(y / x);
785:        th = atan(yy / x0);
798:                gi1 += atan(yy / (xx + j));
805:            th1 = atan(yy / xx);
809:            th2 = atan(si / sr);

amos/amos.h
3363:            t1 = fabs(atan(yy / xx));
4821:                    ang = atan(zthi / zthr);

It might be worthwhile checking if any of those can be replaced by atan2.

[steppi]

Thanks for pointing these out @WarrenWeckesser. Replacing with atan2 sounds good.