OTF2 reader using core reader

[zoreoo]

Basically ready for review. A few notes:

1. The original OTF2 reader added everything into a list (and a dictionary for metrics) and then created a DataFrame based on those lists. With the core reader implementation, I changed everything so that each event is added to the core reader as each event is visited.
2. There may now be metric columns with all NaN data, as opposed to the past implementation that avoided it. I will see what I can do with this.
3. Metrics were a bit tricky, so I hope I didn't mess anything up there.
4. When initializing the core reader, I was not 100% confident in what I put for the start and stride values--but we can look at this.
5. I will need to update comments.