Review the generation code of DCT dictionary

[y-iwao]

Issue Description

The DCT dictionary which is generated by the code of spm-image looks slightly different from examples found on the web.
https://seinzumtode.hatenadiary.jp/entry/20171123/1511468787

Examples found on the web look more symmetric than ours.
We should review the generation code of DCT dictionary or we should find a library which can generate DCT dictionary.

[sutkss]

I fixed dct problem. Now, we get the correct dct dictionary.
I attached the visualization of fixed dct dictionary