set variables in templates to default values

[haraldschilly]

many examples, e.g. from physics, some with variables that have no value set. the goal of the ticket is to parse the code and look for variable names. if they appear, add them above the snippet with some initialization.

[haraldschilly]

here: 2c0a83d and b9949b7

in cocalc, a first idea of that is implemented here: sagemathinc/cocalc@2ec9f3e