Added global iteration counter for the metric x values

[AndreaCossu]

This PR adds a global iteration counter which starts from zero and it is increased by 1 at every training or eval iteration.
The counter is managed by the PluginMetric class. All metrics have been updated to use the global counter.
I also updated the examples with the recent StreamForgetting modifications.

The main modifications the user will perceive are:

• longer distances between a metric x value and the next one (depending on how many iterations have been performed between the two)
• larger absolute values for the x values (total number of iterations)

If you agree with this system we can merge the PR.

This closes #375

[vlomonaco]

it looks good to me! Feel free to merge when the actions complete.
Let's only make sure the doc is clear about what the x axis means.

[coveralls]

Pull Request Test Coverage Report for Build 709321480

• 13 of 52 (25.0%) changed or added relevant lines in 11 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.008%) to 75.959%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
avalanche/evaluation/metrics/confusion_matrix.py	0	1	0.0%
avalanche/evaluation/metrics/accuracy.py	3	5	60.0%
avalanche/evaluation/metrics/loss.py	3	5	60.0%
avalanche/evaluation/metrics/mac.py	1	4	25.0%
avalanche/evaluation/metrics/disk_usage.py	0	4	0.0%
avalanche/evaluation/metrics/gpu_usage.py	0	4	0.0%
avalanche/evaluation/metrics/ram_usage.py	0	4	0.0%
avalanche/evaluation/metrics/cpu_usage.py	0	5	0.0%
avalanche/evaluation/metrics/timing.py	0	5	0.0%
avalanche/evaluation/metrics/forgetting.py	0	9	0.0%

Totals
Change from base Build 709286349:	-0.008%
Covered Lines:	6474
Relevant Lines:	8523

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💛 - Coveralls