Warn on errordef override

[HDembinski]

Closes #930

• Documentation is added to the iminuit.cost module which explains that errordef should not be set manually when the builtin cost functions are used.
• Minuit now warns the user if Minuit.errordef is set if also the cost function has an errordef attribute, but the value set via Minuit.errordef still overrides the Cost.errordef attribute.