Add yaml-based Model loader?

I would like to retire discretizers.disk and replace it by a generic config file format. Something like

type: StationaryModel
operator:
    type: LincombOperator
    operators:
        - L1.mat
        - L2.mat
    coefficients:
        - diffusion[0]
        - diffusion[1]
rhs: F1.mat
output_functional: C.mat
products:
    l2: l2.mat
    h1: h1.mat
parameters:
    diffusion:
        shape: 2
        range: [0.1, 10]

I haven't thought about the details yet, but the general idea would be to have a generic file format, which can be used to load arbitrary Models from disk. Any comments, @pymor/pymor-devs? I think one might also argue that the same could be achieved with few lines of Python code and that we actually do not need such functionality.

If nobody disagrees, I would start by moving discretizers.disk to the graveyard.

Start by reading the existing discretizers.disk module and the proposed YAML example in this issue. Clarify the generic model-loading format and whether it should replace the module or be declined in favor of Python code. Done means the design decision and scope are agreed before implementation begins.