Get Dirichlet dof values

[miguelmaso]

This PR adds an overload of get_dirichlet_dof_values(f::FEFunction) which forwards to get_dirichlet_dof_values(get_fe_space(f)).

Motivation: Other related functions (e.g., get_free_dof_values, get_cell_is_dirichlet) already accept a FEFunction. Adding this method makes the interface consistent.

Closes #1187

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.32%. Comparing base (9cf59d7) to head (2514b2b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1186   +/-   ##
=======================================
  Coverage   89.31%   89.32%           
=======================================
  Files         212      212           
  Lines       27013    27015    +2     
=======================================
+ Hits        24128    24130    +2     
  Misses       2885     2885           

Flag	Coverage Δ
drivers	0.00% <0.00%> (ø)
extensions	0.00% <0.00%> (ø)
tests	89.32% <100.00%> (+<0.01%)	⬆️

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