TRestMetadata: undefine = operator method overload

[nkx111]

It will cause double free problems if we use = or copy constructor for metadata classes. The reason is that the data member fElement as a pointer will also be copied. And when we delete the two metadata classes, the same pointer will be deleted twice. For example:

TRestDetectorGas *g=new TRestDetectorGas("server","Xenon-TMA 0.1Pct 10-10E3Vcm") // initialize a metadata
{TRestDetectorGas gg = *g;} // make a new copy and delete.
delete g;  // this delete operation will cause seg.fault

In this PR I undefined the = operator. Now the second line doesn't work, and avoids this problem.

Now to copy metadata or to use the object(instead of pointer), one can use Clone(), or reference:

TRestDetectorGas* g=new TRestDetectorGas("server","Xenon-TMA 0.1Pct 10-10E3Vcm") 
TRestDetectorGas* gg = (TRestDetectorGas*)g->Clone();
delete g; 
delete gg;

TRestDetectorGas *g=new TRestDetectorGas("server","Xenon-TMA 0.1Pct 10-10E3Vcm")
{TRestDetectorGas& gg = *g;}
delete g;

[jgalan]

Hi @lobis, it is great you took your time to review and approve this PR. However, we should wait till the pipeline is green. So I just requested a second review.

[jgalan]

I have created a new PR at detector library to try to fix the pipeline rest-for-physics/detectorlib#29