Fix two warnings (possible bugs)

[stevengj]

This PR fixes two compiler warnings, which may indicate bugs:

Fixes warning:

    ReadGDSIIFile.cc:622:19: warning: cast from 'libGDSII::BYTE *' (aka 'unsigned char *') to
          'libGDSII::WORD *' (aka 'unsigned short *') increases required alignment from 1 to 2
          [-Wcast-align]
            WORD W = *(WORD *)Payload;

Rather than perform a pointer cast, just explicitly load two bytes and shift them to make a 16-bit word. This has the additional advantage of being more portable, because we can specify the endian-ness with which we read the data. ( I used a little-endian order for backward compatibility with x86 machines. It's not clear to me whether the GDSII format specifies a particular interpretation of bitfield indices, so that endianness matters here?)

Also fix warning:

    ReadGDSIIFile.cc:654:50: warning: adding 'libGDSII::BYTE' (aka 'unsigned char') to a
          string does not append to the string [-Wstring-plus-int]
           *ErrMsg = new string("unknown data type " + DType);
                                ~~~~~~~~~~~~~~~~~~~~~^~~~~~~
    ReadGDSIIFile.cc:654:50: note: use array indexing to silence this warning
           *ErrMsg = new string("unknown data type " + DType);

The std::string + char method does not apply to string literals, which are just pointers. Instead, I just leave a placeholder character X in the string and overwrite it with DType.