add function to normalize

[gidden]

starting to add a function to normalize all data to a specific time value

[coveralls]

Coverage increased (+0.1%) to 84.604% when pulling d8b74b5 on gidden:normalize into fcb81fe on IAMconsortium:master.

[gidden]

we can probably rework this to normalize to any specific subset of data, just works for time at the moment

[danielhuppmann]

looks good, but a few minor changes, please.

And just for kicks: why not (also) have an API like df.normalize(year=2005, inplace=True)? Instead of cols, which might be difficult if you want to support normalizing on multiple columns.

[gidden]

Ok @danielhuppmann all suggested changes made!

[danielhuppmann]

thanks @gidden, looks great! do you want to add it to the release notes for completeness?

[gidden]

done!