Automatically detect linear fitting problems

[francisco-dlp]

It'll be useful to add an is_linear attribute to Parameter so that, if at fitting time all parameters are found to be linear it fits using least squares instead of non-linear least squares. This should dramatically increase the fitting speed in those cases.

TODO

[ ] Add is_linear attribute to all components.
[ ] Add linear least squares to fitters
[ ] Automatically use linear least squares when the problem is found to be linear
[ ] User Guide documentation
[ ] Docstring documentation
[ ] Tests

[tjof2]

See #1453 and #1462.

[thomasaarholt]

I've taken this up again as of this week. The fitting algorithm works, but not yet with convolution. I'd really enjoy it if someone would check out the PR (#1462), where I've merged the linear_parameters PR.

[tjof2]

See #2422 to replace the above links