Support `ArrayType`.

[1uc]

We're missing support for ArrayType, i.e. a way of creating the HDF5 datatype for arrays.

[machsix]

probably something similar to this? instead of char[StrLen] but T[Len]

template <size_t StrLen>
class AtomicType<char[StrLen]>: public DataType {
  public:
    inline AtomicType()
        : DataType(create_string(StrLen)) {}