historical: Swap Ledger and Beancount formatters in the class hierarchy.

This makes sense for a couple of reasons:

* The Beancount formatter has "less features" than the Ledger formatter, so
  this is a more "logical" organization of the hierarchy anyway. Note how
  this eliminates the need for the BeancountFormatter.__init__ override to
  turn off Ledger features.

* Any future work will probably be focused on the Beancount formatter, so
  this reduces the amount of code you have to understand and hold in your
  head to do that.

oxrlib/commands/historical.py

@@ -71,19 +71,15 @@ class Formatter:
         )
 
 
-class LedgerFormatter(Formatter):
-    COST_FMT = '{{={}}}'
+class BeancountFormatter(Formatter):
+    COST_FMT = '{{{}}}'
     PRICE_FMT = ' @ {}'
 
     def price_rate(self, from_amt, from_curr, to_curr):
         if self.price_rates is None:
-            rates = self.cost_rates
+            return None
         else:
-            rates = self.price_rates
-        return rates.convert(from_amt, from_curr, to_curr)
-
-    def can_sign_currency(self, code):
-        return len(babel.numbers.get_currency_symbol(code)) == 1
+            return self.price_rates.convert(from_amt, from_curr, to_curr)
 
     def normalize_rate(self, rate, prec=None):
         if prec is None:
@@ -172,26 +168,18 @@ class LedgerFormatter(Formatter):
         )
 
 
-class BeancountFormatter(LedgerFormatter):
-    COST_FMT = '{{{}}}'
+class LedgerFormatter(BeancountFormatter):
+    COST_FMT = '{{={}}}'
 
-    def __init__(self, cost_rates, price_rates=None,
-                 signed_currencies=(), base_fmt='#,##0.###',
-                 rate_precision=5, denomination=None):
-        super().__init__(
-            cost_rates,
-            price_rates,
-            (),
-            base_fmt,
-            rate_precision,
-            denomination,
-        )
+    def can_sign_currency(self, code):
+        return len(babel.numbers.get_currency_symbol(code)) == 1
 
     def price_rate(self, from_amt, from_curr, to_curr):
         if self.price_rates is None:
-            return None
+            rates = self.cost_rates
         else:
-            return self.price_rates.convert(from_amt, from_curr, to_curr)
+            rates = self.price_rates
+        return rates.convert(from_amt, from_curr, to_curr)
 
 
 class Formats(enum.Enum):