Calculate continued fractions from square roots

The continued fraction module currently creates the square root of a
number as a continued fraction. This is then used in solution 64.
Calculate convergents of continued fractions

Update solution 65 to use the new convergent function.
Merge branch 'PE-64-65-refactor'
Add problem 66 description
Add pell equation solver
Add solution 66
Merge branch 'PE-66'