[Traits] Export maxSubDomainIndex as a function, not as a static constant

Provide maxSubDomainIndex as runtime information in preparation for a new
traits class that will allow to specify the subdomain count at runtime.
[MultiDomainGrid] Store an instance of the Traits in the grid

If we want to allow traits to influence the grid using runtime information,
that information needs to be stored somewhere, so this patch adds a const
instance of the traits to the grid and provides a second constructor that
takes a traits instance with the runtime information.
[IndexSet] Add setup hook for per-subdomain size containers

The per-subdomain size containers need to initialized at runtime if the
subdomain count is only known at runtime. As this setup is clearly dependent
on the traits-specified strategy, let's keep that setup code local to the
new traits instance on the grid.
[DynamicSubDomainCountTraits] New storage strategy for runtime-specified subdomain count

The new traits class DynamicSubDomainCountTraits allows the user to specify the
maximum number of subdomains at runtime, as a constructor parameter to the traits.
The traits have to be passed explicitly to the MultiDomainGrid constructor in the
second argument of that constructor.

Apart from being able to specify the number of subdomains at runtime, the new strategy
is identical to the ArrayBasedTraits.
[iterateallinterfaces] Extend test to run with all three storage strategies
[iterateallinterfaces] Remove useless, commented code
Merge branch 'feature/dynamic-subdomain-count'

Add a new storage strategy based on ArrayBasedTraits that allows specifying the
maximum number of subdomains at runtime.

* feature/dynamic-subdomain-count:
  [iterateallinterfaces] Remove useless, commented code
  [iterateallinterfaces] Extend test to run with all three storage strategies
  [DynamicSubDomainCountTraits] New storage strategy for runtime-specified subdomain count
  [IndexSet] Add setup hook for per-subdomain size containers
  [MultiDomainGrid] Store an instance of the Traits in the grid
  [Traits] Export maxSubDomainIndex as a function, not as a static constant