MerginProject may not get cleaned up properly

[wonder-sk]

When MerginProject is created, it creates geodiff object and assigns an internal logger to geodiff. This creates a cyclic reference (pygeodiff refs MerginProject, and MerginProject refs pygeodiff) that may keep resources held, which may be a problem if MerginProject object is created many times. See also fix for this in db-sync: MerginMaps/db-sync#79

Also somehow related: MerginMaps/geodiff#190

Code snippet to replicate the cleanup issue:

import pygeodiff

class MerginProjectX:

    def __init__(self, directory):

        def _logger_callback(level, text_bytes):
            self.x()   # not a real function, but refs this object...

        self.geodiff = pygeodiff.GeoDiff()
        self.geodiff.set_logger_callback(_logger_callback)

# this will keep consuming more and more resources
for i in range(50000):
    print(i)
    mp = MerginProjectX("x")
    #mp.geodiff = None    # this helps to break ref cycle
    time.sleep(0.005)