new find_mcs function for exclusion of hydrogens

[JohannesKarwou]

Currently, hydrogens are considered when looking for a common core with the _find_mcs function. This is not ideal, since sometimes common cores are found based on these hydrogens, where it is necessary to turn off more heavy atoms than necessary. Thanks to @jalhackl, we now have an advanced algorithm for finding the CC
An example is shown here for 2cpi -> 7cpi, with the old function on the lower part (ccor mit hdrogens) and the new algorithm on the upper part (ccor ohne hydrogens):

The main advantage of the new algorithm, is that some common cores might be bigger (containing more heavy atoms, we don't care about more hydrogens beeing in the dummy region) and that the error reduces drastically.
I excluded two tests (test_proposed_mutation_terminal_dummy_real_atom_match() and test_find_connected_dummy_regions1()) for the mutation 2CPI -> 7CPI, which do not make sens anymore, I suggest replacing them.

Todo

• Write some tests for the new function (usingl the 2CPI -> 7CPI mutation)

[JohannesKarwou]

@akaupang, found this interesting case, where our current _find_mcs algorithm is not working properly.
With the old algorithm, this common core is found:

Ligand 1	Ligand 2

The position of the methoxy group is changing but according to transformato it's the same common core. With the new algorithm, the correct common core is found:

new Algorithm:

Ligand 1	Ligand 2

I added this system as a test case to our transformato_testsystems repo and included a new test.

[wiederm]

Tests are passing and the code looks reasonable, but haven't checked in detail.