Fix trajopt_sco exprToEigen large heap allocations

[Levi-Armstrong]

This is to address issue #309. It turns out that it was allocating enough space to fill out the sparse matrix as if it was a dense matrix which really defeats the point of using a sparse matrix. This updated so item memory intensive variable is thread local and do not reserve space.

[Levi-Armstrong]

This results in an 15% reduction for what I would consider a large planning problem and 90% reductions in planning time for simple motion planning problem.

Before:

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Benchmark                                                         Time             CPU   Iterations
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BM_TRAJOPT_SIMPLE_COLLISION_SOLVE/real_time                    3204 us         3204 us          217
BM_TRAJOPT_PLANNING_SOLVE/min_time:6.000/real_time            46973 ms        46970 ms            1

After:

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Benchmark                                                   Time             CPU   Iterations
---------------------------------------------------------------------------------------------
BM_TRAJOPT_SIMPLE_COLLISION_SOLVE/real_time               355 us          355 us         1966
BM_TRAJOPT_PLANNING_SOLVE/min_time:6.000/real_time      39786 ms        39785 ms            1