Implement a grid template

[nelimee]

Is your feature request related to a problem? Please describe.
For the moment, we can limit ourselves to regular grids of templates (that will likely represent logical qubits). There is nothing in the code that is explicitly here to build such a grid.

Describe the solution you'd like
Constructing a GridTemplate should be quite easy with the ComposedTemplate class.

Describe alternatives you've considered
Having a specific implementation for a grid, just like StackedTemplate and ShiftedTemplate are not implemented using ComposedTemplate (because it does not provide the necessary features) but rather directly as a subclass of Template with a specific behaviour. Turns out that a grid should be describable quite easily with a ComposedTemplate, so this alternative has not been explored.