make code more general to incorporate other internal and external meshes and boundary conditions for them fix bug where loads/ prescribed displacements could only be positive
Closes #50
restructuring:
- new folder Sources/+specimen with subdirectories: +internal contains internal meshes (plate, ...); +external contains function which reads meshes from external files (so far gmsh import added)
- boundary condition setup subdivided into edge_boundaries (wherein edges can be defined for BCs) and node_boundaries (wherein the user can directly give node sets for BCs)
- deleted redundant files for the plate specimen
the changes/ generalisation necessitated changes in ALL input files, where mesh and boundary conditions are defined