Add Fe110 to metal database

[sevyharris]

Add the Fe110 facet to RMG's database.

Values taken from:

Xu L, Kirvassilis D, Bai Y, Mavrikakis M. Atomic and molecular adsorption on Fe(110).
Surface science. 2018;667:54-65. doi:10.1016/j.susc.2017.09.002

The lattice constant PW91 is given in the paper as a=2.85 Angstrom.
Then surface site density was calculated using the following code

import ase.build
import numpy as np
import ase.visualize
import rmgpy.constants

a = 2.85  # A
surface = ase.build.bcc110("Fe", (3, 3, 3), a=a, periodic=True)

a1 = surface.cell[0]
a2 = surface.cell[1]
unit_cell_area = np.linalg.norm(np.cross(a1, a2))
print(f'Unit cell area:\t{area:0.5f}\tA^2')

site_area = unit_cell_area / 9.0
print(f'Site area:\t{site_area:0.5f}\t\tA^2/site')
print(f'Site density:\t{1.0 / site_area:0.5f}\t\tsites/A^2')

SDEN = 1.0 / site_area * (1e20) / rmgpy.constants.Na
print(f'Site density:\t{SDEN:0.5e}\tmols/m^2')
SDEN = 1.0 / site_area * (1e16) / rmgpy.constants.Na
print(f'Site density:\t{SDEN:0.5e}\tmols/cm^2')