FieldMatrix size methods not constexpr

When trying to compile

static_assert(mat.mat_cols() == vec.size(), "");

for FieldMatrix mat and FieldVector vec gcc-5 failed and reported that mat_cols() is not constexpr since FieldMatrix lacks a constexpr constructor. Compilation failed as well with clang-6, but succeeded with gcc-7.

From dune/common/fmatrix.hh:173

constexpr size_type mat_cols() const { return COLS; }

Since this is a member function it implicitly takes a this-argument of a non-literal type FieldMatrix and thus cannot be constexpr.

FieldVector's size method works fine because its constructor is constexpr.

dune/common/fvector.hh:112

    //! Constructor making default-initialized vector
    constexpr FieldVector()
      : _data{{}}
    {}

Possible solutions:

1. Make FieldMatrix's constructor constexpr. (and possibly do the same for DiagonalMatrix)
2. Make the size methods (mat_cols(), mat_rows()) static

Let me know which (or if any) option is preferred so I can prepare a fix.