Add support for complex-valued dofs

[zjwegert]

When using AD for complex-valued problems (i.e., Gridap#1325), the FESpace of the perturbed quantity must have vector_type<:AbstractVector{<:Complex}. For these cases, to use the DifferentiableTriangulation machinery, we need the DiscreteGeometry to take an FEFunction with complex-valued DOFs.

To handle this case, this PR proposes to define geometries using the real part of the provided FEFunction.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.89%. Comparing base (c30c8c0) to head (0eff769).
⚠️ Report is 41 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   76.83%   76.89%   +0.06%     
==========================================
  Files          31       31              
  Lines        3600     3619      +19     
==========================================
+ Hits         2766     2783      +17     
- Misses        834      836       +2     

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