Added a method to write series with a specific time_precision

I did think about changing the existing WriteSeries method to something with a signature like this:

```func (self *Client) WriteSeries(series []*Series, timePrecision ...TimePrecision) error```

But it didn't feel like a clean solution as you would then need to add multiple checks inside the method to make sure all corner cases are treaded correctly. So I opted for a separate function instead, which also feels a little like doing the same stuff twice but then at least is all clean and tidy :)

This PR is also a fix for issue #3 which described exactly the issue I'm trying to solve.