Add S1 pattern likelihood for alt S1

[zihaoxu98]

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

This PR adds new fields of S1 pattern likelihoods for alt S1, which are "alt_s1_2llh", "alt_s1_top_2llh", "alt_s1_bottom_2llh". They could be useful for s1 single scatter cut.

Can you briefly describe how it works?

For alt S1's calculation, most of the changes are just with a prefix "alt_". The only exception is z, with prefix "alt_s1_".

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

Same as before

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: 89.489% (+0.03%) from 89.463%
when pulling 8077155 on alt_s1_llh
into 9813b30 on master.

[GiovanniVolta]

May I suggest @minzhong98 as a reviewer for this PR? He is an expert on the S1 PL cut

[minzhong98]

This PR looks good to me, thanks @zihaoxu98 !

[dachengx]

I think we still need to discuss whether we need this nice PR and more importantly the cut depends on it. So I would hesitate a bit about when to merge this.

[zihaoxu98]

I agree. No rush on merging this PR