Remove vector holding pointers to child algorithms from Algorithm base class

[Russell Taylor]

It isn't used (except for passing through a cancel notification, which we can probably live without).

[Russell Taylor]

(In [1593]) Removed vector of shared pointers to child algorithms from Algorithm base class. This means that sub-algorithms will disappear once you go out of the scope they were created in - so you'd better retrieve any output properties before that happens. The advantage of this is that temporary workspaces will no longer be kept alive just because a (finished) child algorithm is holding a shared pointer to them in its property collection. Re #285.

[Stuart Campbell]

This ticket has been transferred to github issue ​1133