Fix option parser's long option handling

[cniethammer]

Long option names were only compared to known options based on the length of the option name given as arguments, resulting in strange matches, e.g., '--step' was matching '--steps'.

This resolves the parsing issue in #351

[HomesGH]

Good fix for this part of the issue. Do you also want to fix the other part of the issue with this PR or a separate one?
Additionally:
If no valid option is passed, a segmentation fault occurs. This is due to the order of initialization in MarDyn.cpp. First, the Logger should be inited, then the options parsed etc. However, this might not be trivial as the init of the Logger depends on an option. We could init with the default (i.e. print to stdout) and later switch to another Logger if the respective option is specified. What do you think?

ls1-mardyn/src/MarDyn.cpp

Lines 143 to 157 in 98597e0

	optparse::OptionParser op;
	initOptions(&op);
	optparse::Values options = op.parse_args(argc, argv);
	std::vector<std::string> args = op.args();
	/* Initialize the global log file */
	if( options.is_set_by_user("logfile") ) {
	// Print to file
	std::string logfileNamePrefix(options.get("logfile"));
	std::cout << "Using logfile with prefix " << logfileNamePrefix << std::endl;
	Log::global_log = std::make_unique<Log::Logger>(Log::Info, logfileNamePrefix);
	} else {
	// Print to stream (default: std::cout)
	Log::global_log = std::make_unique<Log::Logger>(Log::Info);
	}

[cniethammer]

I would stay with just fixing the addressed part of incorrect argument handling with this PR.

I will open a separate PR with a fix for the initialisation problem - as this is a different topic.