SeriesBuffer should enforce the backend dtype

This should be applied

diff --git a/src/sgnts/base/buffer.py b/src/sgnts/base/buffer.py
index dd8ce21..f04987b 100644
--- a/src/sgnts/base/buffer.py
+++ b/src/sgnts/base/buffer.py
@@ -182,7 +182,8 @@ class SeriesBuffer:
         else:
             if self.shape != self.data.shape:
                 raise ValueError("self.shape and self.data.shape must agree")
-
+        if self.data is not None:
+            assert self.data.dtype == self.backend.DTYPE
         for t in self.shape:
             assert isinstance(t, int)

But it causes a lot of test failures (which suggests that a lot of silent type casting must be occurring atm)