Add option to distribute Geant4 events with fixed timing

[HenningSE]

What does the code in this PR do / what does it improve?

This PR adds the option to distribute events with a fixed timing instead of using a uniform distribution.

Can you give a minimal working example?

Setting the tracked config argument fixed_event_spacing from the default value false to true will set the event timings to fixed values. The event spacing is controlled by the source_rate argument.

Please include the following if applicable:

• [x] Bump plugin version(s)
• [x] Tests to check the (new) code is working as desired.

[coveralls]

Pull Request Test Coverage Report for Build 9695160107

Details

• 7 of 9 (77.78%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 80.513%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
fuse/plugins/micro_physics/input.py	7	9	77.78%
<!--	Total:	7	9	77.78%	-->

Totals
Change from base Build 9695042116:	-0.03%
Covered Lines:	2607
Relevant Lines:	3238

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💛 - Coveralls