Create data format for mathml source files

[physikerwelt]

Create a format for mathml source input files.
For tex there is the following input format
https://github.com/TU-Berlin/pmmlc/blob/master/test/files/extended.json
note that the hash can be used to detect wrong file encodings, which is a common problem when dealing with mathml input files.
One possible solution nis to add an extra key to the input format e.g.

[
  {
    "inputhash": <hash of input/>,
    "input": <json encoded mathml>,
    "format": "pmml"
  }
]