Valgrind reports leak in ScoringFunction

[envidian]

==195708== 120 bytes in 5 blocks are definitely lost in loss record 580 of 1,478
==195708== at 0x4849013: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==195708== by 0x216113: ScoringFunction::ScoringFunction(scoring_function_choice, std::vector<float, std::allocator > const&) (scoring_function.h:50)
==195708== by 0x206D06: Vina::set_forcefield() (vina.cpp:456)

Submitted below as a potential fix. Please review.

#262