Add gmxapi::Workflow.

Add gmxapi::Workflow.

Hold a description of simulation work to be performed. Work description
is a dependency graph that allows a Context implementation to configure
and launch an API Session to satisfy data flow constraints. Initial
implementation just provides command-line type input for mdrun in the
form of a TPR filename. A more complete implementation would look more
like the "work specification" described in
https://doi.org/10.1093/bioinformatics/bty484

For more discussion on a full implementation, refer to
https://github.com/kassonlab/gmxapi/issues?q=milestone%3Agmxapi_workspec_0_2+

Refs #2585

Change-Id: I2c9bb99d88a11893201489757453ef25bc35a52