Merge branch 'diag_stat_bars' into for-merge

merge springs and static solver

Merge branch 'diag_stat_bars' of github.com:kutsjuice/Flexia into diag_stat_bars

feat(flexia): add time dependent forces

feat(6bar): add sinusoidal_func for bd4.forces

Merge remote-tracking branch 'origin/master'

feat(body): add time to body forces and build mass matrix separately

implement parmech

fix(flexia): fix incorrect body boundaries for animation

fix(visualization): fix aspect ratio

feat(examples): add control to crankshaft example

fix(solvers): retore legacy cros! method

sync

feat(theory): add natural frequencies description

add pics

sync april progress

init

lesson-4 update

fix lesson-2

fix lesson-2