Error rework

[mnottoli]

The error system presented some issues, some of which would have been a problem in the future:

• code duplication (error stored in 4 different data structures)
• unavailability of the error when the other data structures are not available (for instance when deallocating)
• incomplete error propagation

The error system has been reworked to fix the issues:

1) New code for the error handling has been added:

• Errors are stored in a derived ddx_error_type, which contains a long message and an error flag
• The errors can be updated by calling the subroutine update_error, this will append a message to the error log and sum 1 to the error flag
• The errors can be checked by calling check_error, in case of a flag different from zero, the error message will be printed and the program stopped

2) The error is now correctly propagated, if a subroutine returns an error, then the caller code takes care of updating the error message and returning.

3) The errors have been decoupled from the other data structures.

• Previously there was some code duplication as the errors were stored in params, constants, workspace and state. Now all the Fortran API routines expect an error variable and the other data structures do not contain the error.
• The ddx_cinterface exports a C function for getting a pointer to the ddx_error_type object. All the C API functions expect a pointer to the ddx_error_type as well.
• The pyddx_classes file was changed as little as possible. The pointer to the ddx_error_type object is stored in the Model class. All the calls to the C API functions get the pointer to the error. The throw_if_error function checks if the error contained in the model is zero, if not, an exception is raised. The throw_if_error function of the State calls the throw_if_error function of the Model.

[mnottoli]

@mfherbst It is not urgent, you can check this once you are back from your travel. I will also run the psi4 tests to make sure that everything works.

[mnottoli]

Psi4-ddX tests are passing.

[mnottoli]

Hi Michael, thanks for the great work. I pushed a review commit. All the tests and the python tests are passing on my notebook. I also ran the psi4-ddX tests and those are passing as well.