Gee

This PR addresses several small but important issues that I discovered when using GEE on a real project.

Here is a summary of the changes:

* Fixed a bug that was causing the residuals to become permanently centered after one call to `center_residuals` (even for subsequent calls to `resid`).

* Force the time variable to always be float64.  Strange things were happening when it was passed in as an integer type.  Also the default was an integer type.

* Fixed a bug that was causing the parameters component of fit_history to contain only the final parameter estimate.

* Fixed a bug that was causing the number of iterations in the results summary to always display as 3.

* Have fit_history contain more useful information.

* Fixed a bug that was causing the user-requested covariance type to be ignored.

* Rename `_cluster_list` to `cluster_array` : the new name is more descriptive and lacks the leading underscore since this function is helpful for users.

* Rename `row_indices` to `group_indices` (more descriptive).

* Added convenience functions to access residuals and centered residuals split by group.

* Added a few tests

* Minor docstring fixes

Note: it looks like I accidentally made some of my dotplot commits to my gee branch.  You can ignore those for this PR (or tell me how to switch them please).