Reverse orientation crashes dnaplotlib

[castillohair]

Using 703c734ad2ce2aa2199cfde9181bb8b8c5803e7a (current master), the following script:

import dnaplotlib as dpl
import matplotlib.pyplot as plt

p1 = {'type':'Promoter', 'name':'p1', 'fwd':True}
c1 = {'type':'CDS', 'name':'c1', 'fwd':True}
design1 = [p1, c1]

# Create the DNAplotlib renderer
dr = dpl.DNARenderer()

# Redend the DNA to axis
fig, ax = plt.subplots()
start, end = dr.renderDNA(ax, design1, dr.SBOL_part_renderers())
ax.set_xlim([start, end])
ax.set_ylim([-15,15])
ax.set_aspect('equal')
ax.set_xticks([])
ax.set_yticks([])
ax.axis('off')

plt.savefig('test_promoter.png', dpi=200)

only works when the fwd property in lines 4 and 5 are set to True, but fails if any of them are set to False with the following error message:

Traceback (most recent call last):
  File "test_dnaplotlib.py", line 74, in <module>
    start, end = dr.renderDNA(ax, design1, dr.SBOL_part_renderers())
  File "C:\Users\casti\Anaconda3\lib\site-packages\dnaplotlib\dnaplotlib.py", line 2295, in renderDNA
    start = part['start']
KeyError: 'start'

It seems that the code introduced in #14 is what causes this error. This makes sense, as 'start' and 'end' are not necessarily properties of all parts. Removing this piece of code restores the proper behavior.