Implement Solution class

[ebrahimebrahim]

Add Solutions to openlifu-python, referencing the matlab code for what needs to go into them, and including the needed serialization functionality.

This is just putting together the class mostly as a data structure with serialization -- all the analysis functionality we will worry about later in #104 and #105.

Here is the definition of Solution in the matlab code.

Additional note from Peter:

fus.treatment.Plan.m (equivalent to openlifu.Protocol) is also a useful reference, as it also has some of the planning responsibilities that go into programmatically generating the Solution

[ebrahimebrahim]

There are existing solution json files we can also use to drive implementation of the solution class. We should include tin this issue the ability to save the array-like data structures associated with a solution. To a more appropriate file format than mat, perhaps npz or xarray's preferred format.

We also need a functionality in Database to create a solution, generating unique IDs based on timestamps