added check for length of value passed into EnergyFilter

[shimwell]

Motivated put this thread on the forum where a user was able to make an EnergyFilter with a single value. In the docstring the energyfilter requires at least a pair of numbers to work.

This PR adds a check on the EnergyFilter value length to ensure it is a minimum value of 2.

Description

Please include a summary of the change and which issue is fixed if applicable. Please also include relevant motivation and context.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks for the improvement!