v0.21.0

Significant bug-fixes and feature additions:

• Added an inverse method to id_map
• Added a method to dump geometry to a Pandas DataFrame
• Added Pseudoinverse to the SDynPy Matrix class
• Added ability to save coordinate information to a Rattlesnake specification file
• Added functionality to read Sierra/SD matrix and degree of freedom map files
• Added functionality to read FRF data from Sierra/SD modal FRF runs
• Added significant substructuring documentation
• Added a more complete list of Exodus elements
• Added Vold-Kalman filter and Digital Tracking filters to TimeHistoryArray objects
• Added a reader for Rattlesnake's upcoming MIMO sine environment data
• Added ability to plot arrows of differing sizes with plot_coordinate
• Changed Frequency-Based Substructuring methods to use pinv instead of solve for singular matrices
• Add block diagonal method to combine TransferFunctionArrays
• Improved Frequency-Based Substructuring tests
• Added mimo_inverse to Shock Response Spectra
• Added ability to ignore compensation pulses in Shock Response Spectrum calculations
• Added aliases to the top-level namespace for Shock Response Spectrum tools
• Added a Nonlinear System object SystemNL that mirrors some of the functionality in the linear System object
• Added duffing oscillator, elastic pendulum, and polynomial mass/stiffness/damping class methods to help create nonlinear systems
• Added a zero-early-time FFT method