Serialization of root-level information in YAML

[ischoegl]

Problem description

The addition of YAML serialization of Cantera objects (#984) currently does not allow for the addition of base-level fields within the YAML output. Likewise, the original information from root-level fields that exist in YAML is not preserved. While a fix for the latter is proposed in #881, the former was commented on in #984 but remains to be completed (i.e. akin to the the --extra option of ck2yaml.)

A related question pertains to how to access information from Python for information that was attached at the base level of the YAML file. Specifically, Python's Solution.input_data retrieves input that is specific to the phase, which does not cleanly map to the YAML structure. #881 contains an access routine for description, but leaves access to other root-level fields unaddressed (Edit: implementation was too trivial to leave unaddressed - now proposed as a new Solution.extra property in #881.).

Edit: #1014 implements the conversion of Python dictionaries to C++ AnyMap, which enables the addition of custom information to internal C++ AnyMap variables.