formatter: add BankFormatter subclass

Problem: The AccountingFormatter class has good overarching methods
for printing the results of a query to the flux-accounting database,
but there is some functionality specific to viewing bank information
from the database that could use its own subclass.

Add a new subclass called BankFormatter, which contains unique methods
for viewing bank/user information in hierarchical and parsable formats
with the view-bank command. Remove the helper functions previously
defined in bank_subcommands.py in favor of using this new subclass. Add
a --fields optional argument to allow the user to customize which
columns they want to be returned when looking at a row of information
for a bank.

src/bindings/python/fluxacct/accounting/bank_subcommands.py

@@ -155,8 +155,8 @@ def view_bank(conn, bank, tree=False, users=False, parsable=False, cols=None):
         select_stmt = f"SELECT {', '.join(cols)} FROM bank_table WHERE bank=?"
         cur.execute(select_stmt, (bank,))
 
-        # initialize AccountingFormatter object
-        formatter = fmt.BankFormatter(cur)
+        # initialize BankFormatter object
+        formatter = fmt.BankFormatter(cur, bank)
 
         if tree:
             if parsable:

src/bindings/python/fluxacct/accounting/formatter.py

@@ -13,7 +13,7 @@
 
 
 class AccountingFormatter:
-    def __init__(self, cursor):
+    def __init__(self, cursor, error_msg="no results found in query"):
         """
         Initialize an AccountingFormatter object with a SQLite cursor.
 
@@ -26,7 +26,7 @@ def __init__(self, cursor):
 
         if not self.rows:
             # the SQL query didn't fetch any results; raise an Exception
-            raise ValueError("no results found in query")
+            raise ValueError(error_msg)
 
     def get_column_names(self):
         """
@@ -102,6 +102,18 @@ class BankFormatter(AccountingFormatter):
     out banks/sub-banks in a hierarchical format and lists of users under banks.
     """
 
+    def __init__(self, cursor, bank_name):
+        """
+        Initialize a BankFormatter object with a SQLite cursor.
+        Args:
+            cursor: a SQLite Cursor object that has the results of a SQL query.
+            bank_name: the name of the bank.
+        """
+        self.bank_name = bank_name
+        super().__init__(
+            cursor, error_msg=f"bank {self.bank_name} not found in bank_table"
+        )
+
     def as_tree(self):
         """
         Format the flux-accounting bank hierarchy in tree format. The bank passed