Simplify running simulations

[boriskaus]

The current way to perform a simulation is a bit complicated, and not easily customisable.

With this PR, we simplify that. The basic idea is that we have a single 2D code, MTK_GeoParams_2D that runs the main simulation:

Grid, Arrays, Tracers, Dikes, time_props = MTK_GeoParams_2D(MatParam::Tuple, Num::NumParam, Dike_params::DikeParam); 

The input are the material parameters (from GeoParams) and two structures in which we specify the numerical parameters and the dike parameters.

Within MTK_GeoParams_2D we have a number of callback functions:

        # Add new dike every X years -----------------
        Tracers = MTK_inject_dikes(Grid, Num, Arrays, Mat_tup, Dikes, Tracers, dike, Tnew_cpu)
        # --------------------------------------------

        # Update time-dependent properties -----------
        MTK_update_TimeDepProps!(time_props, Grid, Num, Arrays, Mat_tup, Dikes)
        # --------------------------------------------

        # Visualize results --------------------------
        MTK_visualize_output(Grid, Num, Arrays, Mat_tup, Dikes)
        # --------------------------------------------

        # Save output to disk once in a while --------
        MTK_save_output(Grid, Arrays, Tracers, Dikes, time_props)
        # --------------------------------------------

        # Optionally Update arrays (such as T) -------
        MTK_update_Arrays!(Arrays, Grid, Num)
        # --------------------------------------------

        # Display output -----------------------------
        MTK_print_output(Grid, Num, Arrays, Mat_tup, Dikes)
        # --------------------------------------------

They are now set to default parameters (many are empty), but can be overwritten in a user-code as:

import MagmaThermoKinematics.MTK_GMG_2D.MTK_print_output
function MTK_print_output(Grid::GridData, Num::NumericalParameters, Arrays::NamedTuple, Mat_tup::Tuple, Dikes::DikeParameters)
    println("$(Num.it), max. temperature = $(maximum(Arrays.Tnew))")
    return nothing
end

An example of how this works is given in examples/MTK_GMG_2D_example2.jl and models a case underneath Unzen:

[aelligp]

For the longterm support it might be good to update the deps to the newest release of Makie.jl (v.0.20.x)