test: Add as many tests for the functionality of `diffpy.srmise` as possible

[stevenhua0320]

Problem

Since at the time of initial py2 migration, we have discovered that this package has no test, so making migration to py3 process is brittle and we made some mistake that is very hard to discover at first glance. If we could have some test on the functionality, we could quickly pinpoint and issue and accelerate the workflow.

Proposed solution

Add as many tests for each class of methods as possible.