[Shear.BySector] IndexError when direction_bin_labels is given #636

Describe the bug
Shear.BySector fails with an IndexError when custom direction_bin_labels are passed. plot_shear_by_sector() reads the sector bin edges from the index labels of the wind rose data using a regex (plot.py, ~line 1977), so any label that isn't of the form '0.0-90.0' breaks it.

To Reproduce

import brightwind as bw
data = bw.load_csv(bw.demo_datasets.demo_data)
bw.Shear.BySector(data[['Spd80mN', 'Spd60mN', 'Spd40mN']], [80, 60, 40], data['Dir78mS'],
                  direction_bin_array=[0, 90, 130, 200, 360],
                  direction_bin_labels=['northerly', 'easterly', 'southerly', 'westerly'])

IndexError: list index out of range in plot_shear_by_sector.

Expected behaviour
The labels are used for display only. The bin edges should come from direction_bin_array (or the default sector bins), not be parsed from the labels.

Found while working on #408.