Add a compile-time option to support clang on OS X 10.9.

Set Pism_USE_TR1 to "ON" if your compiler provides C++ shared pointers
in the std::tr1 namespace and you use "#include <tr1/memory>".

Set Pism_USE_TR1 to "OFF" if you compiler *does not* provide shared
pointers in the std::tr1 namespace (i.e. they are in std) and you
use "#include <memory>".

"ON" is the default, to support older compilers and because some newer
compilers support both "#include <memory>" and "#include <tr1/memory>".
Trying to fix the Ross pre-processing script. See #236.
Remove PetscScalar, PetscReal, PetscInt, etc. See #237.

This change was prompted by a compatibility issue between SWIG 2.0.11
and clang on OS X 10.9. The issue should be resolved once SWIG 3.0.0 is
released (sometime soon), but getting rid of PetscScalar and others in
PISM's API seems to be a good idea anyway.

Now that PISM's API uses standard C++ data types (int, double) in its
API, Python wrappers work with clang, and all tests pass on OS X
10.9.

(Should work with gcc too, but I still need to test that.)