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:

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

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

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