Fix assertion error in normal distribution

[Butakus]

Hi,

This is a follow-up of #66, that fixes #67.
I submitted a new PR in case we prefer to fix the CI first, and then fix the assertion error in a different way than the one I propose here:

I added a check for non-positive standard deviations, which raises an exception when initializing the plugin.
Some tests were setting the std to 0.0. I just moved those values to something really small (1e-12) which is roughly equivalent.

[codecov]

Codecov Report

❌ Patch coverage is 78.94737% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../easynav_bonxai_maps_manager/BonxaiMapsManager.cpp	0.00%	2 Missing ⚠️
.../easynav_navmap_maps_manager/NavMapMapsManager.cpp	0.00%	2 Missing ⚠️
...er/src/easynav_costmap_localizer/AMCLLocalizer.cpp	85.71%	1 Missing ⚠️
...zer/src/easynav_navmap_localizer/AMCLLocalizer.cpp	85.71%	1 Missing ⚠️
...asynav_costmap_maps_manager/CostmapMapsManager.cpp	0.00%	1 Missing ⚠️
.../easynav_simple_maps_manager/SimpleMapsManager.cpp	0.00%	1 Missing ⚠️

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