Fix minor problem in quasistatic solver

[vhirtham]

The trial state calculation used aEigen::MatrixXd to store the Hessian which caused larger problems to crash with bad_alloc. I changed the type to Eigen::SparseMatrix<double>.

[codecov[bot]]

Codecov Report

Merging #266 into develop will not change coverage. The diff coverage is 100%.

@@           Coverage Diff            @@
##           develop     #266   +/-   ##
========================================
  Coverage    90.86%   90.86%           
========================================
  Files          326      326           
  Lines        10906    10906           
========================================
  Hits          9910     9910           
  Misses         996      996

Flag	Coverage Δ
#integrationtests	66.45% <100%> (ø)	:arrow_up:
#unittests	96.24% <ø> (ø)	:arrow_up:

Impacted Files	Coverage Δ
nuto/mechanics/tools/QuasistaticSolver.cpp	91.22% <100%> (ø)	:arrow_up:

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