Enable vectorized calculations

[elainethale]

Per https://github.com/mcabbott/AxisKeys.jl documentation, it seems like maybe performing operations like .+ when the dimension names do not line up will throw an error (which would be good). Thus:

• Forward such operators to ParamArray
• Write tests to ensure that they work as expected in both positive (success) and negative (failure) senses
• Similarly, finish testing that read_param works completely as expected (see TODO in that function)

E.g., a long time ago I noted that we should be able to do calculations like customers.gamma .* transpose(model_data.omega) .* customers.d, which would save us a decent amount of code and fidgeting (i.e., for loops and indexes)