[CAS] arc sine of a complex number

[parisse]

"Use i for factoring" is more restrictive than what complex mode does. For example, if complex mode is on, sqrt(x^2) returns x instead of |x|. On the HP port of giac, some functions like ln were returning errors if the argument would return a complex answer (like domain error on the TI) in non real mode. This was later removed, but I did not check everything, and functions that are not used often like asin/acos have the check.

For the mode switch, my experience with the hp49/50/40 convinced me it was not such a good idea because it requires manual mode switchs after. For example if you make a parametric plot on the 50g, you end up in complex approx mode, and you must switch back to real exact for normal CAS operations (there are shortcuts to do that but non-geeks will not learn them).