Reimplement Clausen function

[alextsui05]

We use the a special case of the Clausen function to compute the energy functional for hyperbolic uniformization of our meshes. Since we can't use GPL code, we can't use libgsl's implementation of the Clausen function. Propose to rewrite using only Boost library:

We can use a series acceleration, which uses the Riemann zeta function to compute the Clausen function. We can approximate the infinite sum by truncating and evaluating efficiently by applying Horner's method.

[alextsui05]

Integrated the Clausen function from the clausen repo, now is just a matter of removing GSL library linkage from all build targets.