Create across function for System

[debeshmandal]

Create a function that returns pairs of all nucleotides that are across from each other.

To do this, we'd have to do something like this:

• during initialisation of a Nucleotide, allocate self.across = -1
• for each nucleotide
• if nucleotide.across != -1: continue
• create list of nucleotides whose base is within a cutoff of its base
• get the closest one
• allocate the nucleotide.index to other.across and vice versa

This will be used for generating CanDo files

[debeshmandal]

Created this for the Nucleotide.make_across()