The class MatchWordStringKernel computes a variant of the polynomial
kernel on strings of same length converted to a word alphabet.

It is computed as

k(x,x')= \sum_{i=0}^L I(x_i=x'_i)+c)^d

where I is the indicator function which evaluates to 1 if its argument is
true and to 0 otherwise.

Note that additional normalisation is applied, i.e.
    k'(x, x')=\frac{k(x, x')}{\sqrt{k(x, x)k( x', x')}}

