pism::NCFile: add the m_ prefix to data members.
pism::NCFile: create a non-virtual interface

...making all virtual member functions protected or private.
Merge branch 'dev' into new-error-handling
NCFile::check() throws an exception if necessary.
[re]move_if_exists_impl() return code should match on all processors.
Use exceptions in PIO.

PIO catches exceptions thrown by NCFile and adds some context for better
error reporting.

(I think it's a bit of a miracle that this compiles and that all 49
tests pass.)