[discretizations] add dim_solution attribute to discretizations
[operators.basic] bugfix
[operators] add IdentityOperator
[operators] add restricted() to ComponentProjection
[operators] add restricted() to Concatenation
[operators] add restricted_to_subbasis to ProjectedEmpiciralInterpolatedOperator
[operators.ei] handle NaNs when using solve_triangular
[reductors] use dim_solution to decide how to project operators
[demos] add plot-error-landscape to burgers_ei