Profile 1d 1 species

[RemDelaporteMathurin]

Proposed changes

This is a quick fix following #685 I hadn't tested it for a single species case.
In HydrogenTransportProblem we have a MixedElement in multispecies but not otherwise. Which means functionspace.sub(..) won't work.

This is not a problem for the discontinuous case since we always create a mixed element, even with only one species.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.38%. Comparing base (db9ae13) to head (c984365).
⚠️ Report is 131 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1015   +/-   ##
========================================
  Coverage    95.37%   95.38%           
========================================
  Files           46       46           
  Lines         2920     2926    +6     
========================================
+ Hits          2785     2791    +6     
  Misses         135      135           

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