Draft: Feature/mapattribute

Towards #89

Serialises map types to bytes according to the format:

<nentries>[(key-bytes)key](val-bytes):val[(key-bytes)key](val-bytes):val...

e.g. the following maps:

  std::map<int, std::string> mapintstr{
      {1, "one"},
      {2, "[some_difficult | ::: ][]{}{}><<<:;?\'\">))(( characters....,,,"},
      {-999, "my default value"}};
  std::map<std::string, int> mapstrint{
      {"one",1},
      {"[some_difficult | ::: ][]{}{}><<<:;?\'\">))(( characters....,,,",2},
      {"my default value",-999}};

are serialized to HepMC Asciiv3 as:

A mapintstr <3>[(4)-999](16):my default value[(1)1](3):one[(1)2](61):[some_difficult | ::: ][]{}{}><<<:;?'">))(( characters....,,,
A mapstrint <3>[(61)[some_difficult | ::: ][]{}{}><<<:;?'">))(( characters....,,,](1):2[(16)my default value](4):-999[(3)one](1):1

Serialization is all handled via other attribute types and they are used like:

//write via evt
// MapStringToIntAttribute is an alias of MapAttribute<StringAttribute, IntAttribute>
evt.add_attribute(
      "mapstrint", std::make_shared<HepMC3::MapStringToIntAttribute>(mapstrint));

// MapIntToStringAttribute is an alias of MapAttribute<IntAttribute, StringAttribute>
gri->add_attribute(
      "mapintstr", std::make_shared<HepMC3::MapIntToStringAttribute>(mapintstr));


//read into evt2
auto mapstrint_read =
      evt2.attribute<HepMC3::MapStringToIntAttribute>("mapstrint")->value();
auto mapintstr_read =
      evt2.run_info()
          ->attribute<HepMC3::MapIntToStringAttribute>("mapintstr")
          ->value();