When only one inset is there, no file should have the `_C` postfix

[adisidev]

Currently, when only one inset is present, upon running

cartogram ../../sample_data/belgium_by_region_since_1995/belgium_by_region_since_1995.geojson ../../sample_data/belgium_by_region_since_1995/belgium_population_by_region_2022.csv --plot_polygons 

from the playground directory, we get the following files:

belgium_population_by_region_2022_C_cartogram.svg  
belgium_population_by_region_2022_C_input.svg 
belgium_population_by_region_2022_input.svg 
belgium_population_by_region_2022_output.svg
belgium_population_by_region_2022_cartogram.geojson 

Further, *_C_* have a grid whereas input and output do not.

[adisidev]

I've mentioned the larger grid issue in #239.

[adisidev]

This issue looks slightly harder than what was initially expected.

Currently, there seems to be some duplication of work going on. Here is what I think is going on:

// Determine the name of the inset
std::string inset_name = map_name_;
if (n_insets() > 1) {
  inset_name += "_" + inset_state.pos();
}
inset_state.set_inset_name(inset_name);

In cartogram_info.cpp , here is where the inset names are determined. When there is not more than one inset, the inset name does not have a postfix.

Funnily enough, later in cart_info.plot_input();, and cart_info.write_svg("cartogram"); (plot_input also uses write_svg underneath), they both set the name to be:

// Figure out combined name
std::string inset_names = "";
for (const InsetState &inset_state : inset_states_) {
inset_names += inset_state.pos();
}

So, ignoring the .geojson file, the files are produced like so:
belgium_population_by_region_2022_C_cartogram.svg --> from cart_info.write_svg("cartogram")
belgium_population_by_region_2022_C_input.svg --> from cart_info.plot_input()
belgium_population_by_region_2022_input.svg --> from inset_state.write_map()
belgium_population_by_region_2022_output.svg --> from inset_state.write_map()

When there is only one inset, we don’t want anything to have _C. So one thing to change here might be how write_svg figures out the combined name. Then, the next thing would be to either disable the writing call from inset_state or that from cartogram_info.

The file generated from inset_state.write_map is called _output and not _cartogram because the file just shows the end state of an individual inset, which may not be a full cartogram yet (since it might only have one state + it still needs to be rescaled in accordance with other insets). For example, the D_output for the US representing Hawaii would have just Hawaii, and then it would not make sense to postfix it with _cartogram.

Thus, I think we should disable the write_map within inset_state when there is only one inset. Consequently, we would also need to make write_svg deal with grids properly. Could you check the case when we add a grid to what write_svg plots? Does the grid still make sense, and is it still correct? Having grid for the whole cartogram would be a helpful addition when plotting stuff with multiple insets.