v0.2.1

This is the first public release on PyPI! Here are the differences from the old version that was part of braingeneerspy.

• SpikeData.from_nest() can take multiple NodeCollections and combine them into one SpikeData
• There is now a SpikeData.from_neo_spiketrains() constructor.
• @TjitsevdM's implementation of the Okun randomization method is available by passing method="poprate_okun" into the randomize methods.
• SpikeData.from_raster() can now handle multiple spikes per bin. It distributes them evenly.
• Miscellaneous bugfixes by @atspaeth and @ashsrobbins.