Species.name is optional, don't fail if it's not present

Improves upon #35

src/readsbml.jl

@@ -168,7 +168,7 @@ function extractModel(mdl::VPtr)::Model
             end
         end
         species[get_string(sp, :Species_getId)] = Species(
-            get_string(sp, :Species_getName),
+            get_optional_string(sp, :Species_getName),
             get_string(sp, :Species_getCompartment),
             formula,
             charge,

src/structs.jl

@@ -78,7 +78,7 @@ Species metadata -- contains a human-readable `name`, a `compartment`
 identifier, `formula`, `charge`, and additional `notes` and `annotation`.
 """
 struct Species
-    name::String
+    name::Maybe{String}
     compartment::String
     formula::Maybe{String}
     charge::Maybe{Int}