Minor reporting fixes

[khaeru]

Two small issues spotted while working on MESSAGEix-Transport:

1. Dask behaviour has changed; see the commit message.
2. Units handling in computations.ratio() was triggering some odd behaviour in pint:

   File "/home/khaeru/vc/iiasa/ixmp/ixmp/reporting/computations.py", line 344, in ratio
   result.attrs["_unit"] = u_num / u_denom
   File "/home/khaeru/.local/lib/python3.8/site-packages/pint/unit.py", line 198, in __truediv__
   return qself / other
   File "/home/khaeru/.local/lib/python3.8/site-packages/pint/quantity.py", line 1270, in __truediv__
   return self._mul_div(other, operator.truediv)
   File "/home/khaeru/.local/lib/python3.8/site-packages/pint/quantity.py", line 115, in wrapped
   return f(self, *args, **kwargs)
   File "/home/khaeru/.local/lib/python3.8/site-packages/pint/quantity.py", line 95, in wrapped
   result = f(self, *args, **kwargs)
   File "/home/khaeru/.local/lib/python3.8/site-packages/pint/quantity.py", line 1231, in _mul_div
   no_offset_units_other = len(other._get_non_multiplicative_units())
   AttributeError: 'Unit' object has no attribute '_get_non_multiplicative_units'

Despite the effort of collect_units() to ensure units are from the same registry, things like this would happen:

kilometer <class 'pint.unit.build_unit_class.<locals>.Unit'>          
megapassenger <class 'pint.unit.Unit'>    

Note the different classes. The explicit re-instantiation prevents this.

How to review

Note that the CI checks all pass.

PR checklist

• ~Add or expand tests.~ Not needed here, this code will be moved upstream shortly.
• ~Add, expand, or update documentation.~ No change in behaviour.
• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #396 (05352ef) into master (95a5ca8) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #396   +/-   ##
======================================
  Coverage    96.8%   96.9%           
======================================
  Files          46      46           
  Lines        5337    5339    +2     
======================================
+ Hits         5171    5174    +3     
+ Misses        166     165    -1     

Impacted Files	Coverage Δ
ixmp/reporting/__init__.py	95.0% <100.0%> (-0.1%)	:arrow_down:
ixmp/reporting/computations.py	98.8% <100.0%> (+<0.1%)	:arrow_up:
ixmp/reporting/utils.py	97.3% <100.0%> (+1.4%)	:arrow_up: