some notes on setting matrix constructors

[njtierney]

These could do some checks like:

• all matrices have the same dimensions
• all matrices have the same age breaks
• a new matrix, 'all' should be constructed and be the sum of all the other matrices (if it doesn't already exist)

I think that there is still scope for a hierarchical class system here:

1. Setting Matrices (a list of matrices with numeric values)
2. Type of setting matrix (contact numbers, contact predictions, ngm, transmission probability)
3. Remainder (setting models objects, setting data frames - unsure if these can be nested somehow under a 'setting' class or something)