Renamed variables and saved filter gain.

Both gain and step were named with k, per Zarchan's derivation.

I changed 'k' to 'n'. I also made the gains (K1,K2, K3) members of the
class so that they are accessible to the caller.
Refactored LeastSquaresFilter

Got rid of stupid callable interface. Put x and in an numpy array.
Moved computation of error to errors() method, where computation of
std is also done.
rename of residual to y

The rest of the equation use the mathematical symbol, why not this
one?