Improve cluster algorithm

[tdegeus]

The algorithm I recently wrote is likely far more efficient : tdegeus/GooseEPM#57
It should be rewritten here a bit because there is no 'tracking' of new pixels

[tdegeus]

• kernel Overhaul cluster algorithm (3d extension needed) #89
• non-periodic implementation Overhaul cluster algorithm (3d extension needed) #89
• 3d implementation : Optimised strides: dealing with edge cases #95
• remove old algorithm (some functions have to still be ported) BREAKING CHANGE Adding labels_centers/center, changing center_of_mass, deprecating pos2img #96