Add function of dependency level of data_types

[dachengx]

What is the problem / what does the code in this PR do

Sometimes we want to know which plugin is higher in the dependency tree than another. This PR adds the tree_levels function of Context, which returns the dictionary containing relative information.

For Example, for a given class with Records, Peaks registered, the tree_levels will return:

{'records': {'level': 0, 'class': 'Records', 'index': 0, 'order': 0}, 'peaks': {'level': 1, 'class': 'Peaks', 'index': 0, 'order': 1}}

Can you briefly describe how it works?

Go through the dependency tree and assign level for the data_types first. The level of a data_type will be higher by at least 1 than its dependencies.

Then the data_types will be sorted by level, class, and index. Where the index is the index of this data_type in provides.

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?

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: 90.302% (-0.03%) from 90.331%
when pulling 4a92b0d on level_in_dependency_tree
into ef2861e on master.