Runtime error vector subscript out of range in debug builds

[hjabird]

Hello,

When running debug builds of exafmm I get runtime errors: vector subscript out of range. This is because memory is allocated in std::vector using reserve rather than resize.

I've corrected this is my own branch of exafmm here. Since its no longer quite the same as this original repo, I can't make a pull request out of it. Sorry.

Thanks for the great work on exafmm.