Interface to property::parse and property::validate should be streamlined

[kpeeters]

This is a problem when property declarations want to insert other property declarations. The correct place to do this is in validate (as that is the last function which gets called before the property is inserted). However, this function accepts a reference to the Ex, and inject_property takes a shared pointer. Right now Indices gets around this by adding the Integer property in the parse function, but that is risky because in principle the validate could fail, and then the Integer property is inserted but the Indices is not. This needs to be cleaned up, unified and documented.

[kpeeters]

validate has now been changed to accept a shared_ptr<Ex>, which solves one of the two problems. However, in order to insert a property, this needs to modify the kernel, so that the Kernel& parameter is no longer const now. Would be cleaner if separated into two functions, but then results of checks done in validate may need to be re-done or stored in the property.