Update `mixedtemplate` format

[MaxvdKolk]

The layout of the mixed scheme template can be simplified

• Avoid using strings to switch methods, e.g. if variable == 'Linear':, this can be overcome by passing the Linear (and others) directly
• Support for specifying a "global" approximation and specific approximations for certain variables, e.g. just specify that variable set n uses approximation x
• Investigate if this can be handled by different __init__, e.g. optional/keyword arguments, or provide a simple wrapper-like class that handles this

[artofscience]

• We also need to be able to define variable/response groups (via construction and/or setters?)

[Giannis1993]

Since we have opened #24, I'm closing this issue.