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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.93%. Comparing base (ce31f37) to head (ae55794).
Report is 111 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
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+ Coverage   66.76%   67.93%   +1.16%     
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  Files         332      336       +4     
  Lines       74071    77025    +2954     
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+ Hits        49456    52326    +2870     
- Misses      24615    24699      +84     

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[godotalgorithm]

MDI functionality has been tested in a separate MDI Mechanic repo [https://github.com/openmopac/MDI_MOPAC_test]. Basic functionality of MOPAC as an MDI engine and plugin has been verified. There may be some issues with plugins if multiple copies of the MDI library are in use and the plugin has loaded a different copy of the library than the calling program, but these issues are not completely understood yet. MOPAC's build options for MDI support may be updated in the future if more control is needed over which copy of the MDI library is in the library's RPATH.