rename `solution`

[brandmaier]

I suggest renaming function solution to estimates or parEstimates or parameterEstimates or something similar. In my view, a solution would be a more general object like the actual fitted model including (mis)fit information etc.

[Maximilian-Stefan-Ernst]

In line with #199, my vote would be for either parameter_estimates() or just parameters().

[Maximilian-Stefan-Ernst]

Closed in favor of #239 ; will be named params.