Allow for custom attributes

[forman]

• The configuration setting attrs can now be used to define dynamically
  computed dataset attributes using the syntax {{ expression }}.

  Example:

  permit_eval: true
  attrs:
    title: HROC Ocean Colour Monthly Composite
    time_coverage_start: {{ lower_bound(ds.time) }}
    time_coverage_end: {{ upper_bound(ds.time) }}

• Introduced new configuration setting attrs_update_mode that controls
  how dataset attributes are updated.

Closes #59
Closes #60

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (29cab0b) 99.83% compared to head (b695007) 99.85%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #61      +/-   ##
==========================================
+ Coverage   99.83%   99.85%   +0.01%     
==========================================
  Files          25       26       +1     
  Lines        1220     1371     +151     
==========================================
+ Hits         1218     1369     +151     
  Misses          2        2              

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