Convert FTRL code to a core datatable class (#1417)

FTRL code has been converted into a core datatable class:
- added `fit` and `predict` methods for training and making predictions;
- added `reset` method to reset the model;
- added getters and setters for all the model parameters;
- the `model` itself can now be returned and loaded as a separate frame;
- some other refactoring and modifications.

The algo itself was also slightly optimized, however, it still gets the same score on Kaggle. 
So we're ok from the accuracy point of view.

c/datatablemodule.cc

@@ -13,7 +13,7 @@
 #include "csv/writer.h"
 #include "expr/py_expr.h"
 #include "extras/aggregator.h"
-#include "extras/ftrl.h"
+#include "extras/py_ftrl.h"
 #include "frame/py_frame.h"
 #include "options.h"
 #include "py_column.h"
@@ -199,7 +199,6 @@ void DatatableModule::init_methods() {
   add(METHODv(expr_unaryop));
   add(METHOD0(is_debug_mode));
   add(METHOD0(has_omp_support));
-  init_methods_ftrl();
   init_methods_aggregate();
   init_methods_str();
   init_methods_options();
@@ -234,6 +233,8 @@ PyInit__datatable()
 
   try {
     py::Frame::Type::init(m);
+    py::Ftrl::Type::init(m);
+
   } catch (const std::exception& e) {
     exception_to_python(e);
     return nullptr;

c/datatablemodule.h

@@ -29,7 +29,6 @@ class DatatableModule : public py::ExtModule<DatatableModule> {
 
     void init_methods();
     void init_methods_aggregate();// extra/aggergate.cc
-    void init_methods_ftrl();     // extra/ftrl.cc
     void init_methods_str();      // str/py_str.cc
     void init_methods_options();  // options.cc
     void init_methods_sets();     // set_funcs.cc

c/extras/aggregator.cc

@@ -45,9 +45,9 @@ namespace py {
 
        // dt changes in-place with a new column added to the end of it
        DataTable* dt_members = agg.aggregate(dt).release();
-       py::Frame* frame_members = py::Frame::from_datatable(dt_members);
+       py::oobj df_members = py::oobj::from_new_reference(py::Frame::from_datatable(dt_members));
 
-       return frame_members;
+       return df_members;
      }
   );
 }