Segfault calling load_bin

[e-carlin]

Calling load_bin on a solver that hasn't run a solve causes a segmentation fault.

For example:

hellweg_test$ python <<EOF
> from rshellweg import solver
> solver = solver.BeamSolver("Hellweg.ini", "input.txt")
> solver.load_bin("all-data.bin")
> EOF
Segmentation fault (core dumped)

I think I have a fix. PR coming shortly...