Add reocurring optimization routines to the SystemSpecification class

As tasks like, getting the shortest rise time on a raster for a given amplitude, or shortest total duration for trapezoidal gradient pulse for a given area are reoccuring tasks, that allways probe the system limits It might be a good idea to let the System class own these functions.

With !4 (merged), the system specification object is a positional argument to all SequenceBaseBlock instantiations, all module implementations can rely on the SystemSpecifications functionality.