make featurization in FeaturizedAtoms a const once Julia 1.8 is released

[thazhemadam]

With Julia 1.8, we'll be able to declare const fields in mutable structs. We should use this to make featurization in FeaturizedAtoms a const, since the featurization assigned to a FeaturizedAtoms object shouldn't change.

diff --git a/src/featurizedatoms.jl b/src/featurizedatoms.jl
index 983cde3..b36050a 100644
--- a/src/featurizedatoms.jl
+++ b/src/featurizedatoms.jl
@@ -14,7 +14,7 @@ Container object for an atomic structure object, a featurization, and the result
 """
 struct FeaturizedAtoms{A,F<:AbstractFeaturization}
     atoms::A
-    featurization::F
+    const featurization::F
     encoded_features::Any
     FeaturizedAtoms{A,F}(atoms, featurization) where {A,F<:AbstractFeaturization} =
         new(atoms, featurization, encode(atoms, featurization))