feat: Add formulas integration

We can now get formulas with method Range.get_formulas.
To set formulas, we can simply set_values with formulas as string. If a string starts with '=' it will be interpreted as a formula.
The method Range.set_formulas now raise an error when used.

sheetfu/model.py

@@ -617,8 +617,11 @@ def set_formulas(self, formulas, batch_to=None):
         Set formulas for the Range.
         :param formulas: 2D array of formulas (size must match range coordinates).
         """
-        # todo: set formula.
-        return formulas
+        raise NotImplementedError(
+            "Setting formulas via Range.set_formulas is not implemented yet.\n"
+            "You can however set formulas string (starting with '=') with the "
+            "method Range.set_values and they will be interpreted as formulas"
+        )
 
     def persist_a1_data_range(self, a1):
         """

sheetfu/parsers.py

@@ -3,7 +3,13 @@
 
 
 class CellParsers:
+    """
+    Class to register every setters and getters from the Google Sheets API.
 
+    Following documentation is essential for understanding what we're doing here.
+    https://developers.google.com/sheets/api/reference/rest/v4/spreadsheets/cells
+
+    """
     @staticmethod
     def get_value(cell):
         value_body = cell["effectiveValue"]
@@ -13,7 +19,12 @@ def get_value(cell):
             .get("numberFormat", {})
             .get("type")
         )
-        for value_type in ['stringValue', 'numberValue', 'boolValue', 'formulaValue']:
+        for value_type in [
+            'stringValue',
+            'numberValue',
+            'boolValue',
+            'formulaValue'
+        ]:
             if value_body.get(value_type) is not None:
                 if format_type in ["DATE", "DATE_TIME"]:
                     return serial_number_to_datetime(value_body[value_type])
@@ -23,6 +34,8 @@ def get_value(cell):
     def set_value(cell):
         if cell is not None:
             if isinstance(cell, str):
+                if len(cell) > 0 and cell[0] == "=":
+                    return {"userEnteredValue": {"formulaValue": cell}}
                 return {"userEnteredValue": {"stringValue": cell}}
             elif isinstance(cell, bool):
                 return {"userEnteredValue": {"boolValue": cell}}
@@ -92,21 +105,24 @@ def set_font_color(cell):
             font_color = hex_to_rgb(cell)
         else:
             font_color = hex_to_rgb('#000000')
-        return {"userEnteredFormat": {'textFormat': {'foregroundColor': font_color}}}
+        return {
+            "userEnteredFormat": {
+                "textFormat": {
+                    "foregroundColor": font_color
+                }
+            }
+        }
 
     @staticmethod
     def get_formula(cell):
-        formula = cell["formula"]
+        formula = cell["userEnteredValue"]["formulaValue"]
         if not formula:
             return ''
         return formula
 
     @staticmethod
     def set_formula(cell):
-        if cell:
-            formula = cell
-        else:
-            formula = ''
-        return {'formula': formula}
-
-
+        if not cell:
+            cell = ""
+        formula = cell
+        return {'userEnteredValue': {"formulaValue": formula}}