Solution.get_total_amount: bufix for mass based units

[rkingsbury]

Fixes a bug in which, due to a stray set of quotation marks, get_total_amount would fail with mass-based units:

   1200             TOT += amt * ion.get_el_amt_dict()[el]  # returns {el: mol per formula unit}
   1202         elif ureg.Quantity(units).dimensionality in (
   1203             "[mass]",
   1204             "[mass]/[length]**3",
   1205             "[mass]/[mass]",
   1206         ):
-> 1207             TOT += amt * ion.to_weight_dict["el"]  # returns {el: wt fraction}
   1209 return TOT

KeyError: 'el'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.31%. Comparing base (a62ea2c) to head (9209926).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
+ Coverage   80.17%   80.31%   +0.13%     
==========================================
  Files          10       10              
  Lines        1448     1448              
  Branches      252      252              
==========================================
+ Hits         1161     1163       +2     
+ Misses        251      249       -2     
  Partials       36       36              

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