Define times and durations consistently throughout client

Define the initial parameters of the Series and SeriesBlock to take in time_ns to avoid precision issues, but expose the time and duration properties to return each in seconds as a float.

Also store duration_ns internally and derive duration from this similarly to keep precision.

In terms of the API, both stream and fetch can take in GPS times as floats, but internally convert them to integer nanoseconds before sending the request to the server. Docstrings are fixed so that it is more clear whether arguments/properties are in nanoseconds or in seconds. In either case, the client will use seconds throughout, but can access the nanoseconds in the {time,duration}_ns properties in the Series/SeriesBlock. Internally, the nanoseconds are used to make server requests and to derive all the second-based properties the client sees.

Fixes #15 (closed) and #16 (closed).