Add Ramsey model

[hartytp]

Add a Ramsey model.

• only covers frequency scans since time scans are just a sinusoid so not much point using a Ramsey model
• the heuristic here only covers the case where w_0 and phi are the unknowns. e.g. we don't have a heuristic for the Ramsey delay / pulse duration. It wouldn't be too hard to add something to cover these, but I can't think of any contexts where we don't have a good initial guess for them / don't know them accurately so it doesn't feel worth the effort of writing, testing and maintaining that code.

Credit: this is largely just pushing code written by @AUTProgram

[hartytp]

Merging this as is. If anyone has feedback (e.g. about parameter names) we can always change it later, but I don't want this to bit rot.