Add default tolerance to Core

As per its documentation, this can be used when computing the new
uniform intermediate geometry representation. I'm working on that right
now, so it's hopefully going to see some use soon.

crates/fj-core/src/core.rs

@@ -2,14 +2,19 @@
 //!
 //! See [`Core`].
 
-use crate::{layers::Layers, validation::ValidationConfig};
+use crate::{
+    algorithms::approx::Tolerance, layers::Layers, validation::ValidationConfig,
+};
 
 /// An instance of the Fornjot core
 ///
 /// This is the main entry point to `fj-core`'s API.
 pub struct Core {
     /// The layers of data that make up the state of a core instance
     pub layers: Layers,
+
+    /// Default tolerance used for intermediate geometry representation
+    pub default_tolerance: Tolerance,
 }
 
 impl Core {
@@ -25,7 +30,13 @@ impl Core {
     }
 
     fn from_layers(layers: Layers) -> Self {
-        Self { layers }
+        let default_tolerance = Tolerance::from_scalar(0.001)
+            .expect("Tolerance provided is larger than zero");
+
+        Self {
+            layers,
+            default_tolerance,
+        }
     }
 }