Fix: handle np.nan

[SuixiongTay]

Summary

This PR addresses a small edge case, exemplified by the input below:

s1.add_amount("Mg+2", f"{np.nan} mg/L")

Output:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[16], line 1
----> 1 s1.add_amount("Mg+2", f"{np.nan} mg/L")

File ~/miniforge3/envs/feynman/code/pyEQL/src/pyEQL/solution.py:1433, in Solution.add_amount(self, solute, amount)
   1415 def add_amount(self, solute: str, amount: str):
   1416     """
   1417     Add the amount of 'solute' to the parent solution.
   1418
   (...)
   1431         Nothing. The concentration of solute is modified.
   1432     """
-> 1433     Q = ureg.Quantity(*_translate_pint_quantity(amount))
   1434     # Get the current amount of the solute
   1435     current_amt = self.get_amount(solute, amount.split(" ")[1])

File ~/miniforge3/envs/feynman/code/pyEQL/src/pyEQL/utils.py:57, in _translate_pint_quantity(amount)
     54     return amount.magnitude, str(amount.units)
     56 match = re.match(r"^\s*([+-]?\d*\.?\d+(?:[eE][+-]?\d+)?)\s*(.*)$", amount)
---> 57 _value, _unit = match.groups()
     58 unit = translate_units(_unit)
     59 return (float(_value), unit)

AttributeError: 'NoneType' object has no attribute 'groups'

[SuixiongTay]

This should address #395 CI's issue.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.26%. Comparing base (6a2279c) to head (a929b5e).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pyEQL/utils.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #417      +/-   ##
==========================================
- Coverage   86.32%   86.26%   -0.07%     
==========================================
  Files          14       14              
  Lines        1865     1871       +6     
  Branches      328      326       -2     
==========================================
+ Hits         1610     1614       +4     
- Misses        209      210       +1     
- Partials       46       47       +1     

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[rkingsbury]

Thank you @SuixiongTay