Mention need to install ellc in inverse problem notebook

[kecnry]

The inverse paper examples notebook uses ellc but does not tell the user that ellc needs to be installed. This should include a note at the top with a link to ellc's GitHub as well as a link to the ellc compute API (which includes notes about the required version, etc).

See phoebe-project/phoebe2#584

[wpr1820]

Hi, sir. I can use phoebe on windows, but I can't run Nelder-Mead Optimizer. It's because ellc cannot be installed, this package seems to be only supported on linux systems.

[kecnry]

@wpr1820 - the optimizers should work with any of the supported compute backends - the example notebook chooses to use ellc because it is quite fast and easy to illustrate how optimizers work, but you should be able to swap it out or use the native phoebe backend.