std::max_element requires forward iterator

[bkpoon]

1. Replace std::max_element with boost::first_max_element in include/boost/graph/bc_clustering.hpp as described in std::max_element requires forward iterator #175.
2. Additionally, include/boost/graph/howard_cycle_ratio.hpp has the same issue and is patched similarly in b9c3b14.

I ran the basic tests. Everything compiles with Xcode 11.1 running on macOS 10.14.6 and it looks like nothing failed, but I'm not sure. I did have to remove disjoint_sets.hpp from include/boost/pending and include/boost/pending/detail for b2 to work with the develop branch.

The testing is how I found the problem with include/boost/graph/howard_cycle_ratio.hpp. There is also a std::max_element in include/boost/graph/planar_detail/bucket_sort.hpp, but it looks like the arguments are acceptable iterators.

git clone https://github.com/boostorg/boost
cd boost
git submodule update --init

<change files in lib/graph>

./bootstrap.sh            <- compile b2
./b2 headers              <- just installs headers
./b2                      <- build compiled components

../../../b2               <- run all tests in lib/graph/test

Let me know if anything else is needed. Thanks!

[bkpoon]

I changed the base from master to develop.