Fix flaky matplotlib-to-plotly conversion of touching bars (negative bargap)

[robertoffmoura]

mpl_to_plotly intermittently crashes when converting figures with touching bars (e.g. plt.hist). The bar gap is computed as bar_start[i+1] - bar_end[i], and for adjacent bars floating-point noise can make this a tiny negative number (e.g. -8.88e-16). Plotly's bargap property only accepts values in [0, 1], so the conversion fails with:

ValueError: Invalid value of type 'numpy.float64' received for the 'bargap' property of layout
    Received value: np.float64(-8.881784197001252e-16)

Reproduces with:

import warnings
import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import numpy as np
import plotly.tools as tls

np.random.seed(0)
plt.hist(np.random.randn(10000), 30)

with warnings.catch_warnings():
    warnings.simplefilter("ignore")
    tls.mpl_to_plotly(plt.gcf())

Fix: get_bar_gap now clamps the gap to [0, 1], so float noise around zero maps to bargap = 0 (touching bars) and genuine gaps are unchanged.