Check for valid values of path-following parameters in logit_solve and related functions

[tturocy]

We have recently exposed some of the algorithm parameters for controlling the numerical continuation in logit_solve, logit_solve_lambda, and logit_solve_branch. At the moment these aren't being checked for being valid values. The initial step length should be positive, and the acceleration factor should always be at least one.

[ankur-kalita]

heyy @tturocy can you assign this issue to me ?... i am working on it