Read two data points instead of one

Currently, we follow logpeek and qpeek's example and read only one data point from each data record/packet. In order to solve #154 (closed), we now read two data points.

Closes #154 (closed)

There are some issues that arise from this merge request:

• The data is not downsampled after being read anymore. This is in response to a problem with the downsampling algorithm. More details can be found in a discussion in this MR.
• A new way of calculating end time of a data record/packet causes #229 (closed). This will be fixed in another MR.