New features + bug fixes

• Doxygen file changed so that all library source is browsable.
• Single-deme version of sample_diploid is commented in detail.
• Added KTfwd::add_mutation, KTfwd::add_mutations and unit test unit/test_sugar_add_mutation.cc. These new functions allow the addition of mutations to diploids in a non-random way, which also means you can fill in a population from external data (e.g., something in a file). These features resolve Issue #28, albeit at a low level.
• Member KTfwd::mutation_base::xtra squeezed into unused space in this type. No extra RAM used, and programs may assign values to that type to represent "stuff", whatever that is.
• KTfwd::extensions::discrete_mut_model got a new constructor allowing * KTfwd::extensions::discrete_mut_model::make_mut to assign values to KTfwd::mutation_base::xtra.
• Added KTfwd::change_neutral, which allows simulations to update the value of KTfwd::mutation_base::neutral and correctly update storage of the affected mutation in all gametes.
• #39 fixed
• #40 fixed