Tests and Fixes

[FlorianReimold]

Core Changes:

• Fixed issue that datagrams would be lost if they received faster than the user polled receiveDatagram()
• Changed API of receiveDatagram() function
  • The std::vector<char> variant does not exist anymore. Use the char* method instead and allocate the vector yourself, if you need to.
  • The function now return error reasons the reason as udpcap::Error
• Added isClosed() function
• Removed hasPendingDatagram() function
• Fixed a memory leak
• Fixed crashes when closing the socket with close()
• Fixed many compiler and clang-tidy warnings

Tests:

• Added GTest as submodule
• Added Tests