Changes in petsc_nonlinear_solver.C for -Wshadow.

Issue #24.
Changes in system.C for -Wshadow.

Issue #24.
Changes in system_io.C for -Wshadow.

Issue #24.
Changes in transient_system.C for -Wshadow.

Issue #24.
Changes in error_vector.C for -Wshadow.

Issue #24.
Changes in perf_log.C for -Wshadow.

Issue #24.
Changes in plt_loader_read.C for -Wshadow.

Issue #24.
Changes in location_maps.C for -Wshadow.

Issue #24.
Changes in plt_loader_write.C for -Wshadow.

Issue #24.
Changes in statistics.C for -Wshadow.

Issue #24.
Changes in tree_node.C for -Wshadow.

Issue #24.
Changes in utility.C for -Wshadow.

Issue #24.
Changes in xdr_cxx.C for -Wshadow.

Issue #24.