v2.1.0

Fast CDR v2.1.0

This minor release includes the following features:

1. Support for IDL's @external annotation (#170)

This minor release includes the following improvements:

1. Mirror workflow for keeping latest release branch up to date (#177)

This release includes the following fixes:

1. Fix warning on VS2017 (#176)
2. Fix missing include directive (#175)
3. Fix for non CWG 1270 revision compliant compilers, e.g. qcc 5.4 (#173)
4. Split optional.hpp into two headers so SWIG can process it (#169)
5. Correctly set CMake minimum required version in tests (#165)

PRs in merged order:
#176, #175, #173, #170, #169, #165, #177