Overload integrators based on model

[eparish1]

To support more model types (e.g., ROMs, Heat Conduction, etc.), I think we should overload the time_integrator w/ the model type, e.g.,

function Newmark(params::Dict{Any,Any},model::SolidMechanics)                                                                                
    integrator_params = params["time integrator"]                                                                                            
    initial_time = integrator_params["initial time"]                                                                                         
    ...

Ideally we could pull some of the information (e.g., num dofs) from the model. This change would allow a Newmark integrator to be defined for different model types.