[BUG] MCCD interpolation

[martinkilbinger]

When calling the function utils.shift_ker_stack in mccd.py, the kernel size lanc_rad is negative for three of the 1262 P3 CFIS tiles. This creates a numpy error. It seems that all PSF moments are -1, which is set as size in line mccd.py:1351.

This error should either be caught earlier, or the lanc_rad variable be set accordingly to a valid number.

[martinkilbinger]

After discussions with @tobias-liaudat this error is due to moments not being correctly estimated by galsim:HSM on certain images.
The variable lanc_rad is the interpolation radius to center the PSFs. In practise lanc_rad = 8 works well.
So an easy fix is to use this default if the input value is too small.

@tobias-liaudat will create a new MCCD version.