template all heap code

[mpadge]

Current hard-coded for <double>, but should be templated to allow for more efficient processing of <int> values.

[mpadge]

Can't really be done in any efficient way, because the different heaps all rely on

template <class T>
class HeapD: public HeapDesc {
    public:
        Heap *newInstance(unsigned int n) const { return new T(n); }
};

all instantiated via

std::shared_ptr <HeapDesc> run_sp::getHeapImpl(const std::string& heap_type) {
    ...
    return std::make_shared <HeapD<BHeap> >();
}

and the object in the shared pointer can NOT itself be templated, so BHeap can not, in this case, be templated, but must be an explicit instantiation. Anyway, values are surely almost always going to be real rather than int, and it'll be easier to implement a separate and explicit <int> version for centrality calculations of #90.