Optimize ODE-system matrices assembly

Change the ODE-system matrices assembly algorithm to yield more sparse
matrices.

The imaginary part of the transient exictation force vector is appliend
inside the ODE system so the exitation force now starts from zero not
from the amplitude value.
Fix bug in stiffness matrix, use sparse matrices

Fix the bug with non-negative off-diagonal ones for every even row in
the extended system stiffness matrix KSys_y leading to wrong results in
the time marching process.

Change the system matrix assembly function to yield sparse system
matrices for memory and numerical efficiency.

Apply a workaround for the eigenvalue calculation of the system by
converting the system matrices to full matrices using full() when
calling MATLAB's eig(A,B)-command.
Change element->system matrix assignment method

The element matrix -> system matrix assignment algorithm via index
vectors is changed to use a more compact scheme using MATLAB's indexing
techniques.
Add finite-element system class

Addition of the class c_sys_fem, which represents the discretized frame
system including all nodes, elements and the system mass and stiffness
matrices.

The class assembles the system matrices by individually adding elements
to the system.

The main script file is altered to use the new class instead of the old
build_sys_matrix-function.
Add modal analysis functionality to sys_fem-class

New methods are introduced to the sys_fem-class for the calculation of
eigenvalues and eigenvectors. Furthermore a specific number of lowest
eigenmodes (or all eigenmodes) can be returned by two get-methods.

The main frame-fem-script is altered to account for the newest additions
of the sys_fem-class.
Add boundary conditions to sys_fem-class

The sys_fem-class is being extended to account for simple homogenous
dirchlet-boundary conditions at specified nodes using a new class
method.

The system stiffness matrix is now manipulated to consider these BCs.

In the main script size this new functionality is tested by simply
clamping the first node of the frame structure.
Add BC-definition for the last node of the frame
Fix wrong order of returned eigenvectors

The returned eigenvector matrix of the c_sys_fem.getModes()-method is
now in the correct order according to the eigenvalue-order (low to max).
Add methods for eigenfrequency calculation
Add plot_nodes-class and remove unused functions

The class c_plot_nodes is introduced, which provides functions for
plotting the nodes of the systems in undisplaced and displaced states
(e.g. for eigenmode visualization).

The MATLAB functions get_mode_shape, plot_mode_shape and plot_nodes are
replaced by that class and thus deleted.
Add common BC methods to sys_fem-class

The mos common BCs for beams (clamped and jointed) can be applied to
selected nodes via the newly introduced methods inside the
sys_fem-class.
Add frame defintion class

A new class c_frame_def is introduced, which creates the nodes vector
for the discretized system of a twodimensional frame structure. The
class needs a list of "parent" nodes where the "parent" beams, specified
by another list connecting certain nodes with beams of different
properties, are discretized by a different amount of elements.

The main script, frame_fem, is altered to account for these changes and
let the new class create the nodes vector neccessary for the discretized
fem-system and system matrices.
Add elementarization to c_frame_def-class

The c_frame_def-class does the addition of beam elements to the
discretized fem-system by itself. The affected commands in the main
script file, which aren't necessary anymore as a result of this, are
deleted.
Add method for multiple element addition
Remove unneeded variable
Use adjacancy matrix for nodes plotting
Simplify adjacency matrix indexing
Add boundary conditions to frame_def-class

Clamped and jointed boundary conditions can be applied to the frame
nodes using the frame_def-class.

The main script frame_fem.m is changed accordingly.
Delete unused functions

With the creation of the two classes sys_fem and frame_def the functions
build_sys_matrix and create_beam are no longer needed and thusly
ommited.
Merge branch 'felix' of git@github.com:langfeldt/frame-fem.git into Timmo