QMCPACK antipattern: Throw runtime error if override isn't implemented.

`SPOSet::makeClone` is a glaring example of this but I believe it is present many other places as well.

When you want to insure that all derived classes implement a method make it a pure virtual method i.e.
```
/** make a clone of itself
* every derived class must implement this to have threading working correctly.
*/
virtual std::unique_ptr makeClone() const = 0;
```
This causes a compile time error as it should, and gives an immediate explanation. Any error that can be stopped at compile time should be.

_Don't do this_
```
/** make a clone of itself
* every derived class must implement this to have threading working correctly.
*/
[[noreturn]] virtual std::unique_ptr makeClone() const;
```
A runtime error will someday occur hopefully its detailed enough make finding the mplementation that threw it easy, but potentially it will even be seen by a user and not at development time. This is not really a runtime error though is it.