Matpower m reader

[CBiasuzzi]

Please check if the PR fulfills these requirements (please use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem ? If so, link to this issue using '#XXX' and skip the rest

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) feature

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)? adds a prototype matpower-model reader and writer for .m files

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

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