Resolve "Add AssembleMassMatrixPlugin"

Closes #90

See merge request mechanics-and-materials/ae108!79

Resolve "Add eigenpair extraction"

Closes #89

See merge request mechanics-and-materials/ae108!78

- SLEPc does not permit resetting problem type to default "0"

Resolve "Set working directory via docker-compose"

Closes #118

See merge request mechanics-and-materials/ae108!107

- simplifies discovery of mounted directory for docker-compose users

Resolve "Add support for nonlinear boundary conditions"

Closes #114

See merge request mechanics-and-materials/ae108!103

- solver ALMM not supported for complex scalar type

- includes tests with invalid definition and/or missing executable
- adds support for testing selection of executables

- based on Basic example
- check output of example in tests

- tests now pass when run with 3 MPI ranks

- supports nonlinear boundary conditions
- note: unit tests fail in MPI-parallel

- some solvers provided by TAO support these constraints

Resolve "Speed up CI using needs keyword"

Closes #117

See merge request mechanics-and-materials/ae108!106