Deprecate variable Trajectory()._t and store the time with the spatial components

[phydev]

When we started coding TrajPy we only thought about processing spatial trajectories like $\vec{r}(t)$ with $\vec{r} = (x,y,z)$. However, we are also interested on providing a tool that can be applied to abstract trajectory spaces, such as blood pressure (BP) over time, where the temporal component should be processed as part of the trajectory.

Therefore, the early implementation for initialising Trajectory() where time is stored in the variable Trajectory()._t is not a good design approach for user experience. We need to deprecate this variable and store all components of the trajectory in Trajectory()._r

Considerations for the back-end:

1. If only 2 dimensions are provided, the code will assume that time must be processed together with the abstract dimension $x$
2. If 3 or more dimensions are provided as input, then the time component should not be used for computing the trajectory features
3. This is likely to break previous tests and examples, careful implementation is required.

[phydev]

We will keep the original design and push this to the user side. It is hard to determine the intended use without introducing several ifelse statements along the code which can slowdown processing. To make it easier for users we can add new features in the GUI and more examples with clinical data.