New dt calculation for IAS15

[dangcpham]

IAS15 old dt calculation method can be slow by giving very pessimistic time steps for certain problems. This new method calculates dt by comparing the second vs. third derivative, naturally giving a timestep:

$$ dt = \frac{y^{(2)}}{y^{(3)}} \cdot \zeta $$

where $\zeta \simeq 6 \times 10^{-2}$ is a constant that can be tuned (but not needed to be fine-tuned).

To use the new method, set sim.ri_ias15.dt_mode=1. The constant $\zeta$ can be set via sim.ri_ias15.zeta, with all the similar caveats as in changing sim.ri_ias15.epsilon.

[dangcpham]

It is now possible to do input/output with ri_ias15.dt_mode and ri_ias15.zeta. Also, backward/forward integration now works (now comparing absolute values of $dt$).

[dangcpham]

Related old parameter $\zeta$ to existing $\epsilon_b$ parameter:

$$ dt = \frac{y^{(2)}}{y^{(3)}} \cdot \zeta \approx \frac{y^{(2)}}{y^{(3)}} \cdot 3 \epsilon_b^{1/7}. $$

The user no longer has to (or able to) specify $\zeta$ directly, but rather can just modify $\epsilon_b$ via r->ri_ias15.epsilon.

[hannorein]

Thanks. I've squashed merged this on the newias branch.