[discretizations] add 'products' to DiscretizationInterface
[docs] improve formatting of field lists
[docs] add global reST substituions
[docs] use intersphinx for references to stdlib and numpy documentation
[docs] improve documentation
[discretizations] a Discretization is linear if all operators _and_ functionals are
[discretizations] cleanup
[discretizations] do not require operator to be linear in StationaryDiscretization
[docs] add documentation