Time units of epoch offset

[ephathaway]

The function add_block in mffpy/bin_writer.py has a offset_us argument that can be given if you want there to be a break between blocks. The documentation says this is given in milliseconds, but I think it needs to be given in microseconds currently. For example, if I want to have a 1 second break between blocks, I would have to specify 1000000 (microseconds) for the offset instead of 1000 (milliseconds).

[ViridianForge]

Closing as addressed by PR #9 - thanks for the keen eye @ephathaway !