Using reshape2 instead of picante

[CChelak]

I found a better function for our purposes inside the package reshape2. They have a melt function that does most of the work for us, but includes the 0's and doesn't flip out with negatives.

Also, I adjusted one of the error messages.

[willpearse]

This is great, thanks. Two tips on comments:

• You don't need to write comments for everything. So, for example, a comment saying "use function barry" isn't necessary - just go ahead and use the function. I try to think of comments as introducing "paragraphs" of code, and that the "sentences" should be reasonably self-explanatory within those paragraphs. It's hard to learn what needs commenting and what doesn't, so don't sweat it too much
• It's good that you're commenting arguments and return values, but it would be better to put them in roxygen2 format. I go through this in the handout that I sent you, but in my commit you can see how I've pushed your comments into that format. Also take a look at nacdb.R for more examples of it.

Good work!