fixed missing subtype.field() bug

[bglaessle]

Generally if there any issues with my changes, I am happy to provide refined fixes
if I get some guidance related to the code design.

• the bug appeared because field() member functions where missing
  in template instantations of copymask, random, gaussian ...
• the fix mostly allows code to be compiled against qdp++, the result should be ok
  for scalar and parscalar builds without threading, but I havent
  run any tests! For threaded builds, it is at least slower, if not wrong
• I cannot predict the behaviour for jit, parvecscalar, etc builds
  additional fixes might be necessary ...
• two issues worth mentioning:
  1. it is an architectural choice how threaded RNG is supposed to behave
     (same result as non-threaded? how is this supposed to be enforced?)
  2. currently "random(ferm[subset])" and "random(ferm,subset)" might
     behave differently (same for gaussian)
• I am unsure about the OSubScalar behaviour, is it correct that
  this is simply supposed to be OScalar again, with no effective Subset?

[bjoo]

Is this still an issue in the devel branch?

[bglaessle]

Commit 36e5dde is already resolved (in devel),
whereas the first commit be03cee can still be applied but I currently don't
know anymore how to trigger the error.
Regardless the previously mentioned "random(ferm[subset])" != "random(ferm,subset)" issue
is still unresolved and threading is definitely slow.
For now do not merge and I will try to reproduce the error within 1-2 weeks.

[bglaessle]

I checked the devel-branch and the Subtype::field() method in include/qdp_qdpsubtype.h is still commented out, e.g.

LatticeFermion f = zero;
random(f[rb[0]]);

does not compile. Using random(f,rb[0]) instead works and seems to be threaded correctly.
The same applies for gaussian and copymask.

There are therefore have several options:

• Enable Subtype::field(), the resulting functions which take OSubLattice<T>& arguments should be/are hopefully implemented correctly. Only field() needs to correctly provide the outer lattice instance.
• Remove function and methods with "OSubLattice&" argument since apparently no one uses them and they are incorrect in the worst case.
• Or, since be03cee is rather hacky and messes up consistency/threading I would recommend against it and maybe try to write a patch based on new suggestions.

BTW: in scalar and parscalar arch the argument of void random(OSubLattice<T> dd) should be a reference. Again the same applies for gaussian and copymask.