SampledData no longer allows a tunable buffer

[AayushSabharwal]

As of MTKv9, parameter values are required to match their symtype. SampledData.buffer is updated to reflect this (in #271) but since it does not have a fixed size, it is marked as Constant and not Tunable.

Changing the definition to @parameters buffer[1:length(buffer)] would solve this issue, except ModelingToolkit.get_u0 scalarizes all default values, which causes varmap_to_vars to take very long for initialization.