Simplification of setup mixed approximation

[artofscience]

The setup of a mixed approximation is rather lengthy currently. This issue addresses the simplification of the initialization of the mixed approximation.

[Giannis1993]

@aatmdelissen has proposed the following structure for the initialization:

# Default initialize all to MMA, Taylor1
mix = Mixed(var_sets, resp_sets, default_intervening=MMA, default_approximation=Taylor1, default_movelimit=...)

# Specify a few sets
mix.set_group(0, 1, intervening=ConLin, approximation=Taylor2, ...)
mix.set_group(1, range(5), approximation=Taylor2)
mix.set_group(1, 2, intervening=Linear)

In this way a default uniform mixed scheme is initialized, and by using the set_group method, specific groups are altered. Intervening variables, approximation, and move limit should be able to set independently, e.g.:

# One variable set, three response sets
mix = Mixed(var_sets, resp_sets, default_intervening=MMA, default_approximation=Taylor1, default_movelimit=...)
mix.set_group(0, [0, 1], intervening=ConLin)
mix.set_group(0, 1, approximation=Taylor2)

Which results in (0,0 - ConLin, Taylor1) and (0,1 - ConLin, Taylor2) and (0,2 - MMA, Taylor1).

[artofscience]

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