ILU: Preconditioner tags now copied in preconditioner classes (no const-ref).

Otherwise one can do pretty bad things, e.g.:
 ilu_tag tag;
 ilu0_precond p(A, tag);  // creates preconditioner
 tag.set_option();        // propagates through to p
 ...
 p.apply(x);              // might crash because option not considered in setup.

The whole preconditioner configuration via tags and setup in the constructor
is pretty bad and requires revision for ViennaCL 2.0.0.