Drift diffusion formulation

[JessicaRehak]

This update adds classes for the calculation of the drift-diffusion term, as well as the drift-diffusion form of the diffusion equation. Includes a major refactor to FixedUpdater that will be applied (in the future) to the other updaters. Now, updaters populate a vector of functions that are automatically called. Building a new updater is as easy as populating the vector and the base class handles the rest.

First step to accomplish #5

[codecov]

Codecov Report

Merging #203 (070fa9e) into dev (79b554e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #203      +/-   ##
==========================================
+ Coverage   98.49%   98.53%   +0.03%     
==========================================
  Files         190      200      +10     
  Lines        3722     3822     +100     
==========================================
+ Hits         3666     3766     +100     
  Misses         56       56              

Impacted Files	Coverage Δ
src/domain/finite_element/finite_element.h	100.00% <ø> (ø)
src/domain/finite_element/finite_element_i.h	100.00% <ø> (ø)
src/formulation/updater/diffusion_updater.hpp	100.00% <ø> (ø)
src/framework/builder/framework_builder.cpp	95.72% <ø> (ø)
...ft_diffusion/drift_diffusion_vector_calculator.cpp	100.00% <100.00%> (ø)
src/domain/finite_element/finite_element.cc	100.00% <100.00%> (ø)
src/formulation/scalar/drift_diffusion.cpp	100.00% <100.00%> (ø)
src/formulation/scalar/drift_diffusion.hpp	100.00% <100.00%> (ø)
src/formulation/scalar/drift_diffusion_i.hpp	100.00% <100.00%> (ø)
src/formulation/updater/diffusion_updater.cpp	100.00% <100.00%> (ø)
... and 18 more

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