Static solvers

Base class for static solvers.

Examples with the use of static solvers are:

• benchmark_FrustrumDC.cpp
• benchmark_MaterialTestConv.cpp
• example_StaticSolvers.cpp
• gsElasticity_Static.cpp
• gsElasticity_Static_XML.cpp
• gsThinShell_Static.cpp
• gsThinShell_Static_XML.cpp
• gsThinShell_WrinklingPerturbedDC.cpp
• gsThinShell_WrinklingPerturbedDR.cpp