test: fix strict-aliasing warning

GCC 4.7 emits the following warning:

.../test/integration/validation/thermodynamics.cpp: In member function ‘void validation::test_method()’:
.../test/integration/validation/thermodynamics.cpp:106:58: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

The warning is due to the conversion between halmd::fixed_vector (aka
std::array) and boost::array. In the absence of support for std::array
by h5xx, we resort to the internal read function using raw pointers.