Change iter argument of nls.multstart for gridstart approach

[padpadpadpad]

Currently iter needs either a single number (for shotgun or lhs) or a vector of numbers (if using gridstart). This could be made more efficient by doing a single number that becomes a vector if using gridstart.

Also can put in some protections if the numebr of iterations is too high.

Similar to https://github.com/padpadpadpad/rTPC/issues/78#issuecomment-3196305124.