removing species NA

One issue with the "surveys" dataset we currently use is that it includes a species abbreviated NA. Using its default arguments, R treats them as missing data.

After talking with Tracy, it seemed a little too complicated to introduce students to the arguments of `read.csv` that we could use to get around it. In particular, as we are leaning towards reading the dataset as the one of the first thing that we do.

This PR removes all observations for the NA species. One of the consequence is that the `record_id` are not continuous anymore and start at 3.

Another alternative would be to use a different code for this species.

Any thoughts or alternative ideas are welcome.