Compress the expression string into one line for bpls

[lizdulac]

Related to PR #4145

Create a function that returns a single line string representation of a derived expression. This is mainly for bpls since one line has a more user friendly format.

ExpressionTree to String Expression:
ADD({sim1/Ux},{sim1/Uy},{sim1/Uz})

$ ./bin/bpls expAdd.bp --show-derived -l
  float    derived/addU  2*{10, 3, 6} = 2.98842 / 28.085
    Derived variable with expression: ADD({sim1/Ux},{sim1/Uy},{sim1/Uz})
  float    sim1/Ux       2*{10, 3, 6} = 0.000224775 / 9.94031
  float    sim1/Uy       2*{10, 3, 6} = 0.0201616 / 9.83318
  float    sim1/Uz       2*{10, 3, 6} = 0.0031 / 9.95085

The current solution is not the best. We do not create derived variables on the read side so the BP5Reader provides an internal function to get the expression string from the deserializer, create an expression on it, and call the pretty-print function on the expression (which is overly complicated). I would still merge this PR like this. We will start creating derived variables on the read side and we can replace the logic with something cleaner at that point. This PR is one step in this direction since it moves the string conversion on the Expression class.

[lizdulac]

Translating an ExpressionTree object instead of an ASTNode object to a single line string expression allows for expressions with the same operator to be merged.

Example:
ASTNode to String Expression:
+( +( {sim1/Ux},{sim1/Uy} ),{sim1/Uz} )

ExpressionTree to String Expression:
ADD({sim1/Ux},{sim1/Uy},{sim1/Uz})