Validate first_step and max_accel parameters values in login_solve and related functions

[lusimeon]

see #459

[tturocy]

Thank you for this. Could you also please consider adding a few tests to check these, and also to check the maxregret parameter? We acknowledge that the test suite does not yet include confirming that errors are thrown on bad arguments, but these issues are part of the process of doing that.

Also, while you are at it, the function names quoted in logit_solve_branch and logit_solve_lambda aren't correct, so if you could clean those up as well this would be super useful.

[lusimeon]

Hi, thank you for your feedback. I've added tests to checks parameters, and correct the mispelled functions names. Seems good for you?

[tturocy]

Thank you - apologies that it has taken so long to come back around to this.

I see only one minor item, which is that the inequality on the test for max_accel is inconsistent - sometimes it's weak, sometimes it's strict. A value of 1.0 is valid for the parameter, so the strict inequality < 1 would be the correct one.

[tturocy]

Merged into master at ad47d54 with many thanks.