Custom constellation scheme

Hi, 

I am currently using `digital.generic_mod` with the included modulation scheme from https://www.gnuradio.org/doc/sphinx-3.7.1/digital/constellations.html (e.g. digital.constellation_8psk, digital.constellation_16qam, etc...) as such:

```

digital_constellation_modulator = digital.generic_mod(

                                        constellation=digital.constellation_bpsk().base(),
                                        differential=False,
                                        samples_per_symbol=sps,
                                        pre_diff_code=True,
                                        excess_bw=excess_bw,
                                        verbose=False,
                                        log=False)

```

However, I would like to use modulation schemes which are not included in digital. I have two questions in that regard:

1) How can I define my own phase-amplitude type modulation schemes such as 4PAM, 64QAM which are not included by default?

2) Can I use digital.generic_mod with FSK type modulations like GFSK? If not how would I use them?

Thanks,

Roman