Major update to Cosmology and HaloDensityProfile modules

The most prominent changes in cosmology are:
- Huge speedup of growth factor, sigma
- Added correlation function
- Minor interface modifications

The most prominent changes in HaloDensityProfile are:
- Object-oriented interface
- Parent class for basic profile operations (density, enclosed mass, surface density, spherical overdensity radii etc.)
- Child classes for NFW, DK14 and user-defined profiles
- Significant interface changes

The most prominent changes in the Concentration module are:
- Much faster computation of DK14 model, due to faster cosmology
- More concentration models

After this pull request, I will try to keep all interfaces stable.