optimizing h5py calling
when throwing invalid_probability_error, first restore things to a consistent state
log the RNG used
put an underscore before internal cython names in rng.pyx
it's fine if the probability ratio is infinity, actually ...
if the Big::m_base gets too large, that is also a good reason to recalculate the inverse...

... as it is an indication that it may soon overflow
drastically reduced the ceperley_determinant_lower_cutoff for when it is safe to do so
fixed RandomNumberGenerator.__repr__()
add_noise_to_matrix_diagonal() function