make max_stepsize optional and passable

[js850]

No description provided.

[smcantab]

also the target ratio for adapting the stepsize is optionbal and passable
in the SENS implementation, don't remember if I changed it in this
implementation (probably not), if not that should be optional and passable
too

On Tue, Jul 16, 2013 at 12:25 PM, Jacob Stevenson
[email protected]:

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