Refactored log-det and computation framework

- Made a separate directory for linear algebra related things, "shogun/mathematics/linalg". Currently it contains which contains 
-- Linear Operators (linop)
-- Linear Solvers (linsolver)
-- Eigen Solvers (eigsolver)
-- Rational approximation of linear operator functions (ratapprox)

- Moved everything specific to logdet inside ratapprox, rest of the things are general.
- Moved everything specific to logdet from "shogun/lib/computation" to "shogun/mathematics/linalg/ratapprox/logdet/computation", such that the computation framework remains most general.
- Separated job-result, job-result-aggregator, job and computation-engine.