Get rid of temporary `convolve` method

So beautiful name and so local functionality... RIP.
Move division by Nx*Ny into convolution method
Implement `Sashas's hbhmatmul`
Add and use convolve2d method

C-code only, without R caller wrap
Merge pull request #116 from eodus/convolve2d_refactor

Convolve2d refactor (stage 1)