Returns 2D convolution of a and b where the size of c is given by
- shape= 'full'
- returns full 2-D convolution
- shape= 'same'
- same size as a. 'central' part of convolution
- shape= 'valid'
- only parts which do not include zero-padded edges
By default shape is 'full'. When the third argument is a matrix returns the convolution of the matrix M by the vector v1 in the column direction and by vector v2 in the row direction