Improve setting explanation in the guide

[Ericgig]

Description
Completed the guide page of the settings to include missing entries as well as options included in settings.core and settings.compile. Added CoreOptions and CompileOptions to the apidoc and completed the docstrings.

I also removed settings that are no longer used anywhere.

Lastly I changed the default for accept_float compile option.

Presently, if an args is a float it cannot be updated to a complex:

>>> a = qutip.coefficient("A", args={"A": 1.})
>>> a(0, A=1j)
TypeError: must be real number, not complex

Creating a new coefficient work would be needed.

This will certainly cause an issue eventually, but updating it to complex automatically would cause some error with brmesolve spectra: (w > w0) as complex cannot be compared.

Without types, string are not any better than functions so removing them altogether is not ideal.

So I changed it to use complex when no comparison are done and float otherwise.

Related issues or PRs
fix #1524