extend append() for updating existing data

[gidden]

We observed recently that there are situations where we want append() to overwrite existing data. At the moment, an error is raised if data is recalculated. It should be extended to allow:

• overwriting
• warning
• raise an error

[danielhuppmann]

just a thought - wouldn't it make more sense to have this as a new function IamDataFrame.merge(other, ...)?