This is rather big update, ftimings can now be used via a C-Api, see
test/c_test.c for an example on how to use it.

This step lead to a slight overhaul also of the Fortran API, there are
now also a number of ..._node member functions of timer_t that can be
used if cannot or does not want to specify the node of interest via an
explicit chain of names. An example:

Instead of

  type(timer_t) :: timer

  ...

  call timer%print("foo", "bar", "baz")

one can now also do

  type(timer_t) :: timer
  type(node_t) :: node

  ...

  node = timer%get_root_node()
  node = node%get_child("foo")
  node = node%get_child("bar")
  node = node%get_child("baz")

  call timer%print_node(node)

This construction might sometimes be necessary, e.g. if the hierarchy is
very dynamic or if the current provided maximum number of six levels in
the non-_node functions is not sufficient (but think about if you REALLY
need more than six levels..).

This is similarly done on the C-side, there is even no restriction on
the number of levels by using variadic lists. Still, also there _node
functions are provided. All C-API symbols are prefixed with "ftimings_"
in order to avoid name clashes.