[ENH] Format lambda value with underscores and remove legacy backends (#15)

# [CLN] Remove legacy backend options and improve numeric formatting

- Removed "aesara" and "legacy" backends from the `AvailableBackends` enum in `config.py`
- Improved numeric readability by replacing `100000000` with `100_000_000` in the L1 regularization parameter

These changes improve code clarity and maintainability without altering functionality.

Author: Leguark

gempy_engine/API/interp_single/_interp_scalar_field.py

@@ -117,7 +117,7 @@ def _optimize_nuggets_against_condition_number(A_matrix, interp_input, kernel_op
     cond_number = BackendTensor.t.linalg.cond(A_matrix)
     nuggets = interp_input.sp_internal.nugget_effect_ref_rest
     l1_reg = torch.norm(nuggets, 2) ** 2
-    lambda_l1 = 100000000
+    lambda_l1 = 100_000_000
     loss = cond_number - lambda_l1 * l1_reg
     loss.backward()
     kernel_options.condition_number = cond_number

gempy_engine/config.py

@@ -8,10 +8,6 @@ class AvailableBackends(Flag):
     numpy = auto()
     PYTORCH = auto()
 
-    # Legacy
-    aesara = auto()
-    legacy = auto()
-
 
 # Define the paths for the .env files