Create CanDo input file

[debeshmandal]

(fill this in more later)

create a function called System.write_CanDo(fout : str = 'out) to create CanDo input files

Depends on #31 being completed

[debeshmandal]

To do this we need the following information:

For each nucleotide:

• [x] id
• [x] up (3' aka before)
• [x] down (5' aka after)
• [x] across
• [x] base

For each base-pair - important not to count twice:

• [ ] COM of the base pair (e0) (aka midpoint between pos_coms)
• [x] a3 vector (aka e1) [of nucleotide_1]
• [x] a2 vector (aka e2) [of nucleotide_1]
• [x] a1 vector (aka e3) [of nucleotide_1]
• [x] nucleotide 1, nucleotide 2