Integrate MassReaction and MassModel creation from strings

[z-haiman]

Problem description

Please explain:

• what you tried to achieve,
• how you went about it (referring to the code sample), and
• why the current behaviour is a problem and what output you expected instead.

Code Sample

Create a minimal, complete, verifiable example .

# Paste your code here.

Actual Output

Expected Output

Output of mass.show_versions()

# Paste the output of `import mass; mass.show_versions()` here.

[z-haiman]

Fixed