Round in BPM to microseconds per beat conversion

[craffel]

e.g. here: https://github.com/craffel/pretty-midi/blob/master/pretty_midi/pretty_midi.py#L1246

This is how it was done when using the python-midi backend, so it is currently this way for backwards compatibility. mido rounds, which makes a lot more sense: https://github.com/olemb/mido/blob/master/mido/midifiles_meta.py#L137 We should switch to using this convenience function.