Fix several memory leaks. I don't promise to have found them all, but I got the obvious ones, and my framework's memory use is now much more stable.

Also throws an exception when handed an uncastable particle pointer (instead of segfaulting), and has a bunch of const correctness fixed. Most importantly, it can accept a vector of const particle pointers, which is much more convenient in several frameworks including mine. These changes should be checked for mistakes; I didn't mean to include them in this PR but the memory leak fix is rather urgent.