Sync policies type inconsistency

[EsipovPA]

This is more of a question, but I'm ready to open a PR.

While working on #130, I've noticed that some of the Sync policies are declared as struct and others as class. For example

The ApproximateEpsilonTime is declared as class

class ApproximateEpsilonTime : public PolicyBase<M0, M1, M2, M3, M4, M5, M6, M7, M8>

At the same time the LatestTime is declared as a struct.

struct LatestTime : public PolicyBase<M0, M1, M2, M3, M4, M5, M6, M7, M8>

Same inconsistency goes on for other policies, and I did not yet look precisely into all of the class declarations in the C++ code yet. I'm open to embarking on such an investigation.
Should not they all be classes? Or structures? Or is there a reason why it is this way? I find myself more of pro-class guy in this situation. And I am ready to open a PR, if this is a valid way to go. Seems like an easy fix to me. But maybe there are some hiddden issues that require such an approach. 🤷