feat: optional to/from methods on Reflector. (#150)

[m-mcgowan]

Changes:

• Separates the has_reflector concept into has_read_reflector and has_write_reflector to allow allow Reflector types to be just readable, just writable, or both readable and writable.

• Adds test cases. The tests are slightly altered from the original test_reflector implementation. They now use a small class hierarchy (Person and Parent) with fields in each class, which is not supported by default. The Reflector implementation in the tests flattens the hierarchy. Test cases for Reflector implementations with only the to and only the from method are present.

• Adds a paragraph to the docs mentioning the user-facing changes.

Fixes issue: #150

[liuzicheng1987]

Looks good to me. I will have to wait for the code checks to run through, but just by looking at the code changes, it looks fine. Thank you so much for implementing this so quickly.

Just one minor thing: In your write_and_read.hpp, you still have this #if 0 ... #endif section, which appears to be something you meant to remove, but then forgot. Could you just quickly remove this?

[m-mcgowan]

Thanks for catching that. I wanted to implement write_and_read in terms of read and write, but after a few minutes I gave up :-)

Unused code removed.

[liuzicheng1987]

All right, merged. Thank you for your contribution!