add a metadata comparison method

[KaraMelih]

What is the problem / what does the code in this PR do
Some low-level changes might result in a new metadata and it is not always easy to find what was changed.

Can you briefly describe how it works?

for any context st the method can be called as st.compare_metadata(runid, target, compared_meta)

compared_meta can be file path to the old metadata json, a dictionary, or a tuple with new runid, target pair.

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

current_metadata = st.get_metadata(runid, 'peak_basics')
corrupted_metadata = copy.deepcopy(current_metadata)
corrupted_metadata['lineage']['merged_s2s'][2]['gain_model'] = ['to_pe_model', 'v6', False]
corrupted_metadata.pop('strax_version')
st.compare_metadata(runid, "peak_basics", corrupted_metadata)
[out]
> values_changed
	 in ['lineage']['merged_s2s'][2]['gain_model'][2]
		True -> False
> dictionary_item_removed  ->	root['strax_version']

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?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

Coverage: 91.426% (-0.7%) from 92.143% when pulling 2005b92 on KaraMelih:master into 0bc1754 on AxFoundation:master.

[dachengx]

Thanks, @KaraMelih . I added a test to this method. I will merge this PR after the test is finished.