Merge branch 'fix/box-fvgeometry-1d' into 'master'

[fix][fvelementgeometry][box] Replace double bracket with single bracket

For the one-dimensional case the double curly brackets seem to result in
an error because it actually tries to assign an initializer list of size
2 to a GlobalPosition of size 1. For larger FieldVectors the double
bracket operator seems to be required by the compiler.

See merge request !138