Add separator argument for variable modifications

This commit adds the ability for variable modification directives to
define the separator between their modification value and the previous
value.

This argument is only used when the method is `append` or `prepend.

lib/ramble/ramble/language/modifier_language.py

@@ -53,7 +53,9 @@ def _execute_default_mode(mod):
 
 
 @modifier_directive("variable_modifications")
-def variable_modification(name, modification, method="set", mode=None, modes=None, **kwargs):
+def variable_modification(
+    name, modification, method="set", mode=None, modes=None, separator=" ", **kwargs
+):
     """Define a new variable modification for a mode in this modifier.
 
     A variable modification will apply a change to a defined variable within an experiment.
@@ -64,6 +66,8 @@ def variable_modification(name, modification, method="set", mode=None, modes=Non
         method: How the modification should be applied
         mode: Single mode to group this modification into
         modes: List of modes to group this modification into
+        separator: Optional separator to use when modifying with 'append' or
+                   'prepend' methods.
 
     Supported values are 'append', 'prepend', and 'set':
 
@@ -94,6 +98,7 @@ def _execute_variable_modification(mod):
             mod.variable_modifications[mode_name][name] = {
                 "modification": modification,
                 "method": method,
+                "separator": separator,
             }
 
     return _execute_variable_modification

lib/ramble/ramble/modifier.py

@@ -150,10 +150,15 @@ def modded_variables(self, app, extra_vars={}):
                 else:
                     prev_val = ""
 
+                if prev_val != "" and prev_val is not None:
+                    sep = var_mod["separator"]
+                else:
+                    sep = ""
+
                 if var_mod["method"] == "append":
-                    mods[var] = f'{prev_val}{var_mod["modification"]}'
+                    mods[var] = f'{prev_val}{sep}{var_mod["modification"]}'
                 else:  # method == prepend
-                    mods[var] = f'{var_mod["modification"]}{prev_val}'
+                    mods[var] = f'{var_mod["modification"]}{sep}{prev_val}'
             else:  # method == set
                 mods[var] = var_mod["modification"]