This commit gets rid of the -Wdefaulted-function-deleted warning
that popped up in Mutation-derived classes.

It came from the fact that Mutation.h had these definitions:

               |     copy      |      move
----------------------------------------------------
constructor    |   default     |    default
----------------------------------------------------
operator       |   delete      |    delete

whereas the derived classes had the following:

               |     copy      |      move
----------------------------------------------------
constructor    |   default     |    delete
----------------------------------------------------
operator       |   default     |    delete

This meant that the move operator was defaulted in the base class
but deleted in the derived classes, producing the warning (the
discrepancy was introduced in 6615dbb4).

This commit deletes all copy operators, leaving only copy constructors,
which are used in the Clone() method shared by Mutation classes.

Note that in *HT classes, the copy constructor is defaulted but
the classes contain pointers, so there might be a memory issue when
calling the constructor.