Use string values for enum option in configuration file

[dweindl]

Currently only integer values are used, which is hard to read and error-prone. Need string-to-enum. Available for ceres in types.cc.

[dweindl]

https://github.com/Neargye/magic_enum will be helpful here if we move to C++17.

[paulstapor]

Sounds cool, would simplify the code when setting options, reading success/error status, etc. 👍