remove spurious M_PI define in some headers

[tomjnixon]

include/adm/elements/gain_interaction_range.hpp
13:#ifndef M_PI
14:#define M_PI 3.14159265359f

include/adm/elements/position.hpp
13:#ifndef M_PI
14:#define M_PI 3.14159265359f

include/adm/elements/position_interaction_range.hpp
12:#ifndef M_PI
13:#define M_PI 3.14159265359f