Get rows from dataframe (chitralverma#131)

* get rows from dataframe

* rename rowIterator to rows

* add docs

* add Series of Time

* add remaining basic types to row

* more functions for rows

* fix doc lint

* allow creation of struct series

* allow creation of schema from fields

* add struct types to row

* remove unused dependencies

Author: chitralverma

core/src/main/scala/org/polars/scala/polars/api/DataFrame.scala

@@ -15,7 +15,7 @@ class DataFrame private (private[polars] val ptr: Long) {
 
   val schema: Schema = {
     val schemaString = data_frame.schemaString(ptr)
-    Schema.from(schemaString)
+    Schema.fromString(schemaString)
   }
 
   val width: Int = schema.getFields.length
@@ -167,6 +167,22 @@ class DataFrame private (private[polars] val ptr: Long) {
 
   def count(): Long = data_frame.count(ptr)
 
+  /** Provides an iterator to traverse a specified number of rows from the DataFrame.
+    * @param nRows
+    *   number of rows to traverse
+    * @note
+    *   if `nRows` is greater than the total rows in DataFrame then all rows are included.
+    * @return
+    *   Iterator of [[Row]]
+    */
+  def rows(nRows: Long): Iterator[Row] = RowIterator.withPtr(ptr).lazyIterator(nRows)
+
+  /** Provides an iterator to traverse a all rows from the DataFrame.
+    * @return
+    *   Iterator of [[Row]]
+    */
+  def rows(): Iterator[Row] = rows(-1L)
+
   def write(): Writeable = new Writeable(ptr)
 
 }

core/src/main/scala/org/polars/scala/polars/api/JSeries.java

@@ -13,6 +13,8 @@
 import java.util.stream.Collectors;
 import java.util.stream.StreamSupport;
 
+import org.polars.scala.polars.internal.jni.series;
+
 class JSeries {
     final static String EmptyString = "";
 
@@ -65,6 +67,7 @@ static Series ofList(String name, Iterable<Iterable> values) {
             sList.add(thisSeries);
         }
 
-        return Series.ofSeries(name, sList);
+        long[] ptrs = sList.stream().map(Series::ptr).mapToLong(Long::longValue).toArray();
+        return Series.withPtr(series.new_list_series(name, ptrs));
     }
 }

core/src/main/scala/org/polars/scala/polars/api/LazyFrame.scala

@@ -15,7 +15,7 @@ class LazyFrame private (private[polars] val ptr: Long) {
 
   val schema: Schema = {
     val schemaString = lazy_frame.schemaString(ptr)
-    Schema.from(schemaString)
+    Schema.fromString(schemaString)
   }
 
   val width: Int = schema.getFields.length