Use a global GRIDS store

[Rennzie]

• To avoid unsafe usage it requires that Geodesy adds Sync + Send to the Grid trait.
• Remove the GridLoader class in favour of free functions.

Something like this:

static GRIDS: Lazy<Mutex<GridCollection>> =
    Lazy::new(|| Mutex::new(BTreeMap::<String, Arc<dyn Grid>>::new()));

#[wasm_bindgen(js_name = loadGrid)]
// If no data_view then the key is either a URL or an IndexDb key
pub fn load_grid(key: &str, data_view: Option<js::DataView>) -> WasmResult {
   let parse_grid =    //.. Parse and load grid 
   GRIDS.lock().unwrap().add(key, parsed_grid)
}


//... Later in Geo.get_grid()

fn get_grid(&self, key: &str) -> Result<Arc<dyn Grid>>{
   GRIDS.get(key)
}