fix showAtomicLines gaps

[jamesmkrieger]

Without the fix, we get overlapping of chains after the second one onto the second one:

In [1]: import matplotlib.pyplot as plt; plt.ion()
Out[1]: <matplotlib.pyplot._IonContext at 0x7fa8eb592940>

In [2]: from prody import *

In [3]: ag = parsePDB('7kec')

In [4]: atoms = ag.ca

In [5]: showAtomicLines(atoms.getBetas(), atoms=atoms, gap=True)

With the fix, everything is as it should be: