Required fixes

Nobody has claimed this yet.

### ~~Default values not properly declared~~
If the function takes a default value in its arguments, it needs to be specified with `py::arg`. For example of `Vector` constructor,
```
// Constructor
.def(py::init([](py::array_t vec, bool distributed, bool copy_data = true) {
py::buffer_info buf_info = vec.request();
int dim = buf_info.shape[0];
double* data = static_cast(buf_info.ptr);
return new Vector(data, dim, distributed, copy_data);
}), py::arg("vec"), py::arg("distributed"), py::arg("copy_data") = true) // default value needs to be defined here.
```
Note the last line.

### ~~`Vector::getData` not returning an array object~~
In c++, `getData` function returns the memory address of the data array. In python, this should return the reference of the data array. Currently, `Vector::getData` does not do any special care, returning a `float` value as a result.
```
.def("getData", &Vector::getData)
```

### `Vector::get_data` naming
Compared to the function above, this function can cause a confusion. Based on its action, it needs to be renamed as `copy_data` or something.

### ~~(Enhancement) `Vector` and `Matrix` as a buffer protocol~~
`Vector` and `Matrix` should support buffer view, to handler large-size arrays without copying them in memory. Useful references are:
- [python buffer protocol](https://docs.python.org/3/c-api/buffer.html)
- [pybind11 numpy instruction](https://pybind11.readthedocs.io/en/stable/advanced/pycpp/numpy.html)

Based on [pybind11 numpy instruction](https://pybind11.readthedocs.io/en/stable/advanced/pycpp/numpy.html), an example of `Matrix` definition should be:
```
py::class_(m, "Matrix", py::buffer_protocol())
.def_buffer([](Matrix &self) -> py::buffer_info {
return py::buffer_info(
self.getData(), /* Pointer to buffer */
sizeof(float), /* Size of one scalar */
py::format_descriptor::format(), /* Python struct-style format descriptor */
2, /* Number of dimensions */
{ self.numRows(), self.numColumns() }, /* Buffer dimensions */
{ sizeof(float) * self.numColumns(),
sizeof(float) } /* Strides (in bytes) for each index */
);
})
```

Similar definition is possible for `Vector` as well.

### (Enhancement) Supporting slice view of `Vector` and `Matrix`
`Vector` and `Matrix` must support slice view of its data.

Start by locating the Vector and Matrix pybind11 bindings and compare their current APIs with the issue's naming and slice-view requirements. Use the linked Python buffer protocol and pybind11 NumPy references as the design starting point. Done means the remaining naming concern and slice-view support are resolved for both types and verified by relevant tests.

Written by the indexing model from the issue text.