Handle null 'readonly' fields

plot.readonly is nullable in otm1 but not otm2
Enhance and cleanup 'save_user' function

change model -> user for readability
change ambiguous try/except block to conditional block
add documentation
add sanitation logic
fix arity bug in partial application
Add migrator support for config file over args
Stop holding all fixtures in memory

There's no need to hold all fixtures in memory. Change some code around
so that they get read into memory just long enough to be used and released.
Merge pull request #1515 from steventlamb/topic/cleanup_migrator_params

Topic/cleanup migrator params