Adds the option to write solution vectors corresponding to each unknown as binary files or as ascii files (default behaviour).
I/O times are about 100 times faster with the binary format.
Post processing has been modified in order to handle raw binary files.
CI test is rather crude: it merely expands the existing one for the Elasticity in order to do a first of runs with ascii outputs and another set of runs with binary ones. Results of the postprocessing for both binary and ascii outputs are then compared to the expected ones.