added line truncation to pdb_tidy for >80 chars

[ETC-tom]

Stated behaviour of pdb_tidy:

Modifies the file to adhere (as much as possible) to the format specifications.


Expects a sorted file - REMARK/ATOM/HETATM/END - so use pdb_sort in case you are
not sure.

This includes:
    - Adding TER statements after chain breaks/changes
    - **Truncating**/Padding all lines to 80 characters
    - Adds END statement at the end of the file

Will remove all original TER/END statements from the file.

Usage:
    python pdb_tidy.py [-strict] <pdb file>

Example:
    python pdb_tidy.py 1CTF.pdb
    python pdb_tidy.py -strict 1CTF.pdb  # does not add TER on chain breaks

However truncating lines >80 characters is not implemented in the code.
I have added this functionality and a corresponding unit test.

Current behaviour

INPUT

TITLE     THIS IS A VERY LONG LINE WHICH SHOULD BE WRAPPED CORRECTLY OVER MULTIPLE LINES WITH PADDING\n

OUTPUT line remains unchanged

Desired behaviour

INPUT

TITLE     THIS IS A VERY LONG LINE WHICH SHOULD BE WRAPPED CORRECTLY OVER MULTIPLE LINES WITH PADDING\n

OUTPUT

TITLE     THIS IS A VERY LONG LINE WHICH SHOULD BE WRAPPED CORRECTLY OVER       \n
TITLE     MULTIPLE LINES WITH PADDING                                           \n

[ETC-tom]

This behaviour can be evaluated by executing the following commands in terminal

echo "TITLE     THIS IS A VERY LONG LINE WHICH SHOULD BE WRAPPED CORRECTLY OVER MULTIPLE LINES WITH PADDING" > temp.pdb
pdb_tidy temp.pdb