Variables group in `Equivalence` statement do not cover the case with character substring names

[lsafina]

Equivalence statement's elements can include list of variable names, array element names, array names, and character substring names. Currently it is implemented as each its element should be of FASTFortranVariable, which is not the case for character substring names.

This can be fixed in two ways

1. Make FASTFortranSubstring extend FASTFortranVariable
2. Loose the type of variable group making it less restrictive

[lsafina]

doc https://docs.oracle.com/cd/E19957-01/805-4939/6j4m0vn9b/index.html

[NicolasAnquetil]

Character substring are expression of the form S(I:L) ? We could:

• loosen up the type for equivalence statement's elements (to TExpression)
• define a new trait TEquivalenceElement

[NicolasAnquetil]

inviting @uNouss to the discussion

[lsafina]

ignore