Update the constant-pH tutorial

[kosovan]

Fixes #3941

Description of changes:

• add prior knowledge, make equations and units more consistent
• add a flag to run electrostatics with Debye-Hückel instead of P3M
• accelerate tutorial with DH, cell system, non-interacting particle type
• fix ambiguous explanations

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[kosovan]

Some interesting observations on speedup:

• RE.set_non_interacting_type(len(TYPES)) speeds up the calculation of interacting system roughly by a factor of 2.0.
• Replacing P3M with DH makes the calculation with electrostatics almost as fast as without
• If we do reaction in every cycle and integration just in some cycles, we save the amount of runtime roughly proportional to PROB_INTEGRATION. In the previous version we were doing the opposite, so we were wasting some 50% of the reaction samples.

If the non_interacting_type has such an effect on the speed, I think it is desired to give it a fixed and low value, or to set dynamically to the lowest possible value that is available.

Maybe we should consider using some of these tricks also in the tests of the RxMC and cpH algorithms. It is not important to get perfectly correct physics but to ensure that we can still reproduce the titration curve of an interacting system if we change something in the algorithm. What do you think of that? @jngrad @RudolfWeeber

[jngrad]

Maybe we should consider using some of these tricks also in the tests of the RxMC and cpH algorithms.

Good idea, I can have a look at this later this week to speed up the testsuite, and mention it in the docs too.

A couple of tests did not pass CI. Let us know when you are ready with the PR and we'll review it.

[kosovan]

TODO: before this PR is merged, we should change the initial set of parameters back to the ideal system and clear the cells with solutions of the excercises.

[jngrad]

Thanks for the improvements!