Added Equation and MathML datatype

[JR-1991]

This pull request adds two new math types to handle and evaluate equations. The first type is called Equation, representing equations as strings. It then converts them to sympy expressions and extracts all symbols. The second type is the MathML implementation, which works similarly to Equation but serializes to MathML. However, the MathML implementation cannot be serialized into JSON due to its structure since the operations are empty elements that are not supported in JSON.

Examples

To use the new datatypes, the following markdown demonstrates the necessary type names:

### KineticLawEq

- kinetic_law
   - Type: Equation

### KineticLawMathML

- kinetic_law
   - Type: MathML

Both types are used in the same way, yet differ in their respective output. Here is a demonstration how both can be used and serialized:

Equation Type

ds = lib.KineticLawEq(kinetic_law="a*x + b")
print(ds.json())

{
  "id": "5c37c618-adb8-4497-972f-8df4fe4c2088",
  "kinetic_law": {
    "equation": "a*x + b",
    "symbols": [
      "x",
      "b",
      "a"
    ]
  }
}

MathML Type

Important

The resulting XML tree will always use the math tag, no matter what the attribute name is. This is necessary to comply to the MathML schema.

ds = lib.KineticLawMathML(kinetic_law="a*x + b")
print(ds.xml())

<?xml version='1.0' encoding='UTF-8'?>
<KineticLaw id="1aed15ed-7cb7-4de5-aea0-c68fa969dc1e">
  <math xmlns="http://www.w3.org/1998/Math/MathML">
    <apply>
      <plus/>
      <apply>
        <times/>
        <ci>a</ci>
        <ci>x</ci>
      </apply>
      <ci>b</ci>
    </apply>
  </math>
</KineticLaw>

Evaluation

Both data types evaluate upon being called and can thus be used for calculations. This can be done in the following way:

ds = lib.KineticLawEq(kinetic_law="a*x + b")
ds.kinetic_law(a=1, b=2, x=3)

5.0

[haeussma]

Tested the new types and methods on an existing data model. Works as expected.