Add MDI Engine Functionality

[taylor-a-barnes]

This is an in-progress PR to enable MOPAC to function as an MDI engine. Currently, this build allows MOPAC to link against the MDI Library and call the MDI_Init() initialization function if MOPAC was launched with a -mdi command-line option. Basic MDI functionality will require the following:

• Identify the proper location for MOPAC to listen for MDI commands.
• Create a loop to allow MDI to continuously listen for commands from an external driver, until it receives an EXIT command.
• Add an appropriate response for basic MDI commands, with the most important being >COORDS, >CELL, <ENERGY, and <FORCES.
• Enable plugin functionality by adding an MDI_Plugin_init_mopac() function that is callable from the MOPAC shared library.

Status

• Ready for merge