Revisit MDHolder

[eirrgang]

This class should probably be further simplified and maybe refactored in conjunction with an update to the Context concept on the libgromacs side. Note there are also some possibly confusing semantics and naming with the gmxapi::Workflow (not in the public interface, resembling the Python gmx.workflow.WorkSpec) and gmxapi::MDWorkSpec, which is in the public API but differs in important ways from the Python WorkSpec.

gmxapi::MDWorkSpec should probably be renamed to something more specific to its use in MDHolder and undergo some combination of generalization, minimalization, and obfuscation.