Add Electric Field quantity with corresponding units and test.

[crystal-growth]

Adding Electric Field quantity [ElectricPotential / Length] with basic units V/m.

Additional non-SI units are:

• Volts per mil
• atomic unit of electric field = Hartree Energy / electron charge / Bohr radius

[crystal-growth]

Added test case for atomic unit of electric field, should fail until atomic units from PR #324 are merged.

[iliekturtles]

Thanks!