add offset method to compute offsets relative to base-year values

[gidden]

Please confirm that this PR has done the following:

• [x] Tests Added
• [x] Documentation Added
• [x] Name of contributors Added to AUTHORS.rst
• [x] Description in RELEASE_NOTES.md Added

Description of PR

This PR introduces an offset() method which allows a user to compute offsets relative to base-time-periods in the same style as normalize().

    For example, offsetting from `year=2005` will provide data
    *relative* to `year=2005` such that the value in 2005 is 0 and 
    all other values `value[year] - value[2005]`.

    Conceptually this operation performs as:
    ```
    df - df.filter(**kwargs) + padding
    ```

[codecov[bot]]

Codecov Report

Merging #659 (b203f56) into main (c5e8f9c) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #659   +/-   ##
=====================================
  Coverage   94.5%   94.6%           
=====================================
  Files         59      59           
  Lines       5742    5765   +23     
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+ Hits        5431    5454   +23     
  Misses       311     311           

Impacted Files	Coverage Δ
pyam/core.py	94.6% <100.0%> (+<0.1%)	:arrow_up:
tests/test_core.py	100.0% <100.0%> (ø)

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[danielhuppmann]

Thanks @coroa for suggesting the high-performance implementation alternative!

I don't mind adding the function directly for now. If discoverability becomes an issue, we could delegate methods operating only on the timeseries data to a submodule, similar to what I did with the compute module...

[gidden]

Hi both - have updated based on your suggestions. Should be ready.

[coroa]

LGTM

[gidden]

Given positive response here, will go ahead with merge. See #662 for an issue on expanding documentation further.