Rename castNotNull into castToNotNullable and add castToNullable

Author: nikitinas

src/main/kotlin/org/jetbrains/kotlinx/dataframe/api/TypeConversions.kt

@@ -83,11 +83,17 @@ public fun <T> DataColumn<T>.asComparable(): DataColumn<Comparable<T>> {
     return this as DataColumn<Comparable<T>>
 }
 
-public fun <T> DataColumn<T?>.castNotNull(): DataColumn<T> {
+public fun <T> ColumnReference<T?>.castToNotNullable(): ColumnReference<T> = cast()
+
+public fun <T> DataColumn<T?>.castToNotNullable(): DataColumn<T> {
     require(!hasNulls()) { "Column `$name` has nulls" }
     return this as DataColumn<T>
 }
 
+public fun <T> DataColumn<T>.castToNullable(): DataColumn<T?> = cast()
+
+public fun <T> ColumnReference<T>.castToNullable(): ColumnReference<T?> = cast()
+
 // region to array
 
 public inline fun <reified T> DataColumn<T>.toTypedArray(): Array<T> = toList().toTypedArray()

src/main/kotlin/org/jetbrains/kotlinx/dataframe/impl/api/corr.kt

@@ -4,7 +4,7 @@ import org.jetbrains.kotlinx.dataframe.ColumnsSelector
 import org.jetbrains.kotlinx.dataframe.DataFrame
 import org.jetbrains.kotlinx.dataframe.api.Corr
 import org.jetbrains.kotlinx.dataframe.api.cast
-import org.jetbrains.kotlinx.dataframe.api.castNotNull
+import org.jetbrains.kotlinx.dataframe.api.castToNotNullable
 import org.jetbrains.kotlinx.dataframe.api.convertToDouble
 import org.jetbrains.kotlinx.dataframe.api.dataFrameOf
 import org.jetbrains.kotlinx.dataframe.api.getColumnsWithPaths
@@ -41,10 +41,10 @@ internal fun <T, C, R> Corr<T, C>.corrImpl(otherColumns: ColumnsSelector<T, R>):
 
     val index = cols1.map { it.name }.toValueColumn(indexColumnName)
 
-    val cols = cols1.associateTo(mutableMapOf()) { it.path to it.data.convertToDouble().castNotNull() }
+    val cols = cols1.associateTo(mutableMapOf()) { it.path to it.data.convertToDouble().castToNotNullable() }
     cols2.forEach {
         if (!cols.containsKey(it.path)) {
-            cols[it.path] = it.data.convertToDouble().castNotNull()
+            cols[it.path] = it.data.convertToDouble().castToNotNullable()
         }
     }