add dense vector implementation
add a test checking dense vector interface
- rm _vector_type, operator[]
- rename getNRows() to size()
getNRows() was renamed to size()
changed to use inheritance here
define explicitly a copy assignment operator= which takes a scalar value for initializing all entries
- add operator<<  for Vector
- change an argument of Vector::write() from ostream to string (i.e. filename)
unsigned is replaced with std::size_t in Vector
replace spaces with tab for consistency
rename Vector class to DenseVector class
Reuse operator= and [] from valarray.
Merge pull request #18 from endJunction/add-Vector

Reuse operator= and [] from valarray.
Merge pull request #135 from norihiro-w/add-Vector

Add vector