Warn even kernel size instead of halting

[jomy-kk]

Wouldn't it be more friendly if BioSPPy solved for the user the issue of passing an even kernel size when median filtering? Because raising an error halts the program, which isn't necessary since it's not problematic. Here's a proposal to solve the issue and alert the user.

[rafaelscsilva]

Thank you @jomy-kk for noticing this usability issue and proposing a solution, which is greatly appreciated. Instead of raising a Warning, I would suggest using the warnings Python module to achieve this since it prevents halting the program. To ensure good formatting in BioSPPy here's my suggestion:

1. Add the warnings module to the list of imports (preferably right after the compatibility imports)

# built-in
import warnings

2. Change the Warning exception to a UserWarning

warnings.warn("When the kernel is 'median', size must be odd, so the size was decremented by one.")

Let me know if this suggestion achieves the intended purpose of your PR. If so, I kindly ask you to update your PR with these changes.