Check non-positive lone_hits

[dachengx]

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

If lone_hits["right_integration"] - lone_hits["left_integration"] <= 0, later at

straxen/straxen/plugins/merged_s2s/merged_s2s.py

Line 140 in 33992c2

lh["length"] = lh["right_integration"] - lh["left_integration"]

, lh["length"] will be non-positive, so that error will shown later at strax.add_lone_hits:

straxen/straxen/plugins/merged_s2s/merged_s2s.py

Line 145 in 33992c2

strax.add_lone_hits(merged_s2s, lh, self.to_pe, n_top_channels=n_top_pmts_if_digitize_top)

.

Related to #1357.

Can you briefly describe how it works?

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

[coveralls]

coverage: 91.367% (+0.02%) from 91.346%
when pulling ccee83f on check_lone_hits
into 33992c2 on master.