Submodel machinery and improvements

Suggested future of ty files

• Right now ty files still instantiate everything (expensive) without offering a chance for anything else to look at the model parameters cheaply (e.g. Lancet checking parameters).
• Proposal: the ty file should not call the model object to instantiate the model. Instead, it should 'install' the model on topo.sim.model.
• Proposal: Make topo.sim callable, use topo.sim() to instantiate the model (obviously an error will be generated if the model is not specified).
• Proposal: Calling topo.sim.run will automatically instantiate the model if it has not done so already.
• Hopefully this system would allow Lancet / run_batch to look at the model object before any instantiation happens.

Miscellaneous fixes

• Sheet objects should also have properties.

• Models need a sensible __repr__

• Warn if matchcondition defined (and not None) and no projections were created (for that condition)

• Tobias made the following comment in issue #573 :

  The current implementation does not work with SheetSpec properties which have non-string values, as name+=prop fails (name is a string). This is currently the case for spatial frequency sheets.

  There are two ways to fix this:

  1. Properties must always have string values, which would need to be documented. The drawback with this fix is that things like channel=properties['SF'] if 'SF' in properties else 1 would become more slightly more complicated, i.e. channel=int(properties['SF'][2:]) if 'SF' in properties else 1. However, it would have the advantage that we could stick to the current naming convention.
  2. The simplest solution would be changing name+=prop to name+=str(prop). However, then SF sheets would be called LGNOn1, LGNOn2 and so forth. As long as there is only SF, this is fine. However additional "integer-"properties would make it harder to distinguish which sheet is meant, at least from the sheet name. Internally, everything would be still nice because the properties can always be accessed.

• Jim made this comment about the attribute access being over-specified in #573 :

  Right. I wouldn't want to enforce globally unique projection names, but note that incoming projections are guaranteed to have unique names for a given target sheet, so in principle gcal.projections.V1.LGNOn.LGNOnAfferent could be done as gcal.projections.V1.LGNOnAfferent. In practice there may be some other reason to have LGNOn explicitly, which is fine if so.

The distant future

• Functionality to inspect the structure of SheetSpec and ProjectionSpec objects to check the network has one fully connected component.
• Render a Model instance as a model diagram (functionality currently only offered by the Model Editor)