Create truncatedprior.py

fixes #779 
This function leverages the truncnorm distribution from the SciPy library to generate samples from a truncated normal distribution. You can adapt the function to other distributions by changing the import statement and parameters.

bambi/priors/truncatedprior.py

@@ -0,0 +1,19 @@
+import numpy as np
+from scipy.stats import truncnorm
+
+def truncatedprior(mean, std, lower, upper, size=1):
+    """
+    Generate truncated normal distribution samples.
+
+    Parameters:
+    - mean (float): Mean of the normal distribution.
+    - std (float): Standard deviation of the normal distribution.
+    - lower (float): Lower bound of the truncated distribution.
+    - upper (float): Upper bound of the truncated distribution.
+    - size (int): Number of samples to generate.
+
+    Returns:
+    - ndarray: Samples from the truncated normal distribution.
+    """
+    a, b = (lower - mean) / std, (upper - mean) / std
+    return truncnorm(a, b, loc=mean, scale=std).rvs(size)