Created truncatedprior.py

[speco29]

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.

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[tomicapretto]

Hi @speco29, thanks for the interest in contributing, but this is not what is intended by the issue, which I admit, is very poorly described. What we want is something more general that allows users to truncate any prior distribution, using pymc.Truncated under the hood. I'll add more info in the issue.

[speco29]

Thank you for the review @tomicapretto .
I'll dive into it deeper and will correct it.