convert energies to float64 before summing them in analysis script

[gschramm]

Changes in this pull request

• fix floating point precision issues when summing energies of many events

Testing performed

Related issues

• fixes float precision error in avg. energy calc. of analysis script #141

Checklist before requesting a review

• I have performed a self-review of my code
• I have added docstrings/doxygen in line with the guidance in the developer guide
• The code builds and runs on my machine

Contribution Notes

Please read and adhere to the contribution guidelines.

Please tick the following:

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in the ETSI software (the Work) under the terms and conditions of the Apache-2.0 License.