Move YAML writing code from psamm-import to psamm.datasource.native

[spikeliu]

Currently, the code that is responsible for writing YAML model files is embedded in psamm-import. This means that API users will have to copy this code from psamm-import or reimplement it in order to write new model files or convert existing files. Since the YAML-model reading code is already in the main PSAMM package, it would make sense to also include the YAML-writing code.