iburn estimate can fail

[dflemin3]

iburn estimate can fail

[dflemin3]

Current fix: if no roots are found, set iburn to 0

try:
    min_root = np.min(roots)
except ValueError:
    min_root = 0
    warn_msg = "WARNING: Burn-in estimation failed.  iburn set to 0."
    warnings.warn(warn_msg)

autolength.append(min_root)

[dflemin3]

This issue is specific to our current burn-in estimation function, so it's fixed. In #16 I demonstrate that we need to reconfigure how burn-in estimation is done, so future burn-in calculation discussion should go there.