Missing initialization handling

[guidoagapito]

Description
When specula.init() is not called before creating SPECULA objects, the global variables global_precision and default_target_device_idx remain None.
This causes issues when initializing BaseTimeObj and any objects that inherit from it.

Current Behavior
If specula.init() is not called, objects can be created but may behave unexpectedly or fail with confusing errors.
The uninitialized state is not caught early, leading to difficult debugging.

Proposed Solution
A warning is issued when BaseTimeObj detects uninitialized globals, and it falls back to safe defaults (CPU device with single precision).

Alternative Approaches
Raise an error (stricter): Force users to explicitly call specula.init() before creating any objects.

@alfiopuglisi and @FabioRossiArcetri:
Should we maintain the current warning approach or switch to raising a RuntimeError to enforce explicit initialization?

[guidoagapito]

This issue causes problems when running some tests, the ones without specula.init().
This happens only when tests run individually.
For this specific point I opened #475.

[alfiopuglisi]

I propose two possibilities:

1. force the user to call init(). Similar to now, but raise an exception if specula is used without having called init()
2. if the user does not call init(), perform it automatically with sensible defaults (like device=0 and precision=float32), with fallbacks like CPU, etc.

[guidoagapito]

The first option seems clearer to me and requires the user to choose a specific configuration.