Dataframe feature (#361)

* custom value trait

* functions for custom value trait

* custom trait behind flag

* open dataframe command

* command to-df for basic types

* follow path for dataframe

* dataframe operations

* dataframe not default feature

* custom as default feature

* corrected examples in command

crates/nu-protocol/src/value/custom_value.rs

@@ -0,0 +1,42 @@
+use std::fmt;
+
+use crate::{ast::Operator, ShellError, Span, Value};
+
+// Trait definition for a custom value
+#[typetag::serde(tag = "type")]
+pub trait CustomValue: fmt::Debug + Send + Sync {
+    fn clone_value(&self, span: Span) -> Value;
+
+    // Define string representation of the custom value
+    fn value_string(&self) -> String;
+
+    // Converts the custom value to a base nushell value
+    // This is used to represent the custom value using the table representations
+    // That already exist in nushell
+    fn to_base_value(&self, span: Span) -> Result<Value, ShellError>;
+
+    // Json representation of custom value
+    fn to_json(&self) -> nu_json::Value {
+        nu_json::Value::Null
+    }
+
+    // Any representation used to downcast object to its original type
+    fn as_any(&self) -> &dyn std::any::Any;
+
+    // Follow cell path functions
+    fn follow_path_int(&self, count: usize, span: Span) -> Result<Value, ShellError>;
+    fn follow_path_string(&self, column_name: String, span: Span) -> Result<Value, ShellError>;
+
+    // Definition of an operation between the object that implements the trait
+    // and another Value.
+    // The Operator enum is used to indicate the expected operation
+    fn operation(
+        &self,
+        _lhs_span: Span,
+        operator: Operator,
+        op: Span,
+        _right: &Value,
+    ) -> Result<Value, ShellError> {
+        Err(ShellError::UnsupportedOperator(operator, op))
+    }
+}