Remove <vb_epidist> class and methods

[joshwlambert]

The <vb_epidist> class was added to the package to provide a convenient way to handle epidemiological parameters where there are extrinsic (vector) and intrinsic (human) parameters in a single object.

However, it has not found any application thus far (from discussions with package users) and for now is adding unnecessary complexity to the package, both from a maintenance and development perspective, but also for new users coming to the package, who might spend time trying to understand what the purpose of this class is.

Therefore, as discussed in #93 this class and it's method will be removed from the package.

[joshwlambert]

Closing as resolved in #359.