Action:
Fills the sequence file with a specified letter in a defined position
along the DNA chain. The rest of the nucleotides will have the letter
"B" as it was defined in create sequence.
CYCLIZE script assigns the letter "B" for regular BDNA and "A" for all the A-tract positions.
Example:
%seq = &fill_name( \%seq, "C" @cap_site );
This assigns the letter "C" to all nucleotides in position "cap_site" which should be defined in the script.