Support dynamic time-step sizes

[davidscn]

The implemented solvers only support constant time-step sizes. We added asserts to prevent misuse. The time-stepping schemes could theoretically handle dynamic time-step sizes, but within the solvers, we create initially dependent data structures once, such that a dynamic time-step size invalidates these data structures. Examples:

dealii-adapter/source/nonlinear_elasticity/include/nonlinear_elasticity.h

Lines 241 to 250 in dbb25be

	// Coefficients, which are needed for time dependencies
	const double alpha_1 =
	1. / (parameters.beta * std::pow(parameters.delta_t, 2));
	const double alpha_2 = 1. / (parameters.beta * parameters.delta_t);
	const double alpha_3 = (1 - (2 * parameters.beta)) / (2 * parameters.beta);
	const double alpha_4 =
	parameters.gamma / (parameters.beta * parameters.delta_t);
	const double alpha_5 = 1 - (parameters.gamma / parameters.beta);
	const double alpha_6 =
	(1 - (parameters.gamma / (2 * parameters.beta))) * parameters.delta_t;

dealii-adapter/source/linear_elasticity/linear_elasticity.cc

Lines 350 to 351 in dbb25be

	stepping_matrix *= time.get_delta_t() * time.get_delta_t() *
	parameters.theta * parameters.theta;