CorrelateSections turned into class

I didn't like the way CorrelateSections added to Area's errors array
each time correlate_all was called. Instead, an Area makes its own
Correlation class, passing either itself or a bunch of its sections
to the constructor.

Area#errors already pulls errors from each of its sections and returns
the concatenation of those errors plus its own @errors, so now it also
concatenates Correlation#errors (as it also includes Parsable).
Renames correlate_sections.rb to correlation.rb