Provide get_centroids implementation

Author: MaheshGPai

tdigest/include/tdigest.hpp

@@ -143,6 +143,11 @@ class tdigest {
    */
   uint64_t get_total_weight() const;
 
+  /**
+   * @return centroids
+   */
+  vector_centroid get_centroids() const;
+
   /**
    * Returns an instance of the allocator for this t-Digest.
    * @return allocator

tdigest/include/tdigest_impl.hpp

@@ -85,6 +85,11 @@ uint64_t tdigest<T, A>::get_total_weight() const {
   return centroids_weight_ + buffer_.size();
 }
 
+template<typename T, typename A>
+auto tdigest<T, A>::get_centroids() const -> vector_centroid{
+  return centroids_;
+}
+
 template<typename T, typename A>
 A tdigest<T, A>::get_allocator() const {
   return buffer_.get_allocator();