Fix common errors.

[t-karatsu]

Hello. I found two errors building aspa, so suggest source fixes.
Could you check if it is the right fix?

• defined new member 'Vec_value_type' in 'self' namespace.

     460    In file included from ../src/mtl_headers/mtl/matrix_implementation.h:27:
  >> 461    ../src/mtl_headers/mtl/linalg_vec.h:135:21: error: no member named 'Vec_value_type
            ' in 'linalg_vec<RepType, RepPtr, NN>'
     462            while (*i == self::Vec_value_type(0)) ++i;
     463                         ~~~~~~^

I defined Vec_value_type as a member of linalg_vec namespace with reference to other header file src/mtl_headers/mtl/dense1D.h.

• fix narrowing error.

  >> 915    ../src/utils/toolbox/database/HDFDatabase.cc:689:24: error: non-con
            stant-expression cannot be narrowed from type 'int' to 'hsize_t' (a
            ka 'unsigned long long') in initializer list [-Wc++11-narrowing]
     916          hsize_t dim[] = {nelements};
     917                           ^~~~~~~~~

I did a typecast for nelements to 'unsigned long long'.