Fix bug in C version

A wrong term was set, which led to slightly erroneous values.
Thanks to this bug fix, the allowed error ratio is now significantly
lower.
Fix a translation bug in stability

This bug is not spotted by the current validation suite, but it is still
significant, as it makes convergence faster.
Add a computation intensive test case
Make computation of complex invert behave as in FORTRAN

Any other combination unfortunately yields different result, that turn
to a high error ratio for the intensive test case.
Add a utility to dump the textual content of a .grid
Prevent denormalized number when SSE is activated

This leads to dramatic performance improvement without much accuracy
loss.
Merge pull request #1 from serge-sans-paille/fast-and-stable

Fast and stable