CSignal not thread safe

[tklein23]

Running "valgrind --tool=helgrind" on multithreaded shogun binaries gives a lot of warnings related to CSignal:

==19506== Possible data race during write of size 1 at 0x70E2C51 by thread #6
==19506== Locks held: none
==19506== at 0x640EFD7: shogun::CSignal::clear_cancel() (Signal.cpp:130)
==19506== by 0x5E6F5A6: shogun::CLibLinear::train_machine(shogun::CFeatures_) (LibLinear.cpp:80)
[...]
==19506== This conflicts with a previous read of size 1 by thread #7
==19506== Locks held: none
==19506== at 0x5E4FFD5: shogun::CSignal::cancel_computations() (Signal.h:94)
==19506== by 0x61063B6: shogun::CDotFeatures::dense_dot_range_helper(void_) (DotFeatures.cpp:269)
[...]