Functionality for global variables

[evenmn]

In the current version, the global variables are hardcoded. This is not what we want in the future when we get more coupled parameters. There are two kinds of coupled parameters:

1. Parameters that depend on each other within a group (ex. H_ij=A_ij(sigmai+sigmaj)^eta_ij for the Vashishta potential)
2. Parameters that are linked to types rather than groups (ex. charges)

For the first one, we can make a dictionary linking the respective parameters:

intra_coupled_params = {'A': {'target': 'H', 'dependencies': ['A', 'sigmai', 'sigmaj'], 'expression': 'H=A*(sigmai+sigmaj)**eta'},
                                         'sigmai': {'target': 'H', 'dependencies': ['A', 'sigmai', 'sigmaj'], 'expression': 'H=A*(sigmai+sigmaj)**eta'},
                                          'sigmaj': {'target': 'H', 'dependencies': ['A', 'sigmai', 'sigmaj'], 'expression': 'H=A*(sigmai+sigmaj)**eta'}}

and we can loop through this every time the 'global' group is selected. Similarly, the parameters that are linked across the groups can also be addressed in a list:

inter_coupled_params = ['Z', 'alpha', 'sigma']

the current behavior is that the parameter and the type are separated by an underscore (ex. 'Z_H'). However, it might be less risky to separate them by a colon ('Z:H') as some parameters may contain underscores. This should also be implemented for the case without 'global'