Utility Function "isempty" is a Freak Creature

[rdbyk]

1. help page of isempty tells not the whole truth, there are some questionable hidden features (cf. here)
2. in theory isempty should be a very simple and basic utility, but in reality it is really slow, and sadly it is used quite a lot in the code base
3. don't expect C API function scilab_isEmpty to behave like isempty, it is not.
4. undcoumented bells and whistles of isempty are rendering code intransparent

Proposal:

1. rename isempty to isevil (just kidding)
2. avoid all evil and stay with the good forces ..., i.e. for the time being don't use isevil at all
3. in the long run, one may replace isevil by a new, efficent, and simply defined isempty utility