Fix inertia matrix computation

Fix inertia matrix computation so that A11, A12 and A22 matrices are
computed correctly.
Specifically this fix ensures all columns of A12 matrix are computed.
Update symbols to indicate A11 matrix by `Jcomp` in the output file.
Add `pysymoro/inertia.py` file
Refactor code - inertia matrix computation

Move all inertia matrix computation functions from
`pysymoro/dynamics.py` to `pysymoro/inertia.py`.
Update UI call for inertia matrix

Update UI menu call for inertia matrix.
Add `compute_inertiamatrix()` method to `Robot` class.
Modify to return the entire inertia matrix

Modify to return the entire inertia matrix of size [(6+n) x (6+n)].
Update to compute inertia matrix for fixed base robots

Add function `fixed_inertia_matrix` in `pysymoro/inertia.py` to compute
the inertia matrix for fixed base robots.
Update `Robot` class to call the correct function accordingly.