Clean up pylint errors via config options

The majority of pylint gripes are related to Astropy modules with
dynamically generated attributes (units and constants) and short and
quirky variable names (a, b, c, P1). Disable these to get a respectable
pylint score :-) Also document the max line length in case we want to
change it one day.

The alternative to setup.cfg would have been the newer and shinier
pyproject.toml. Both these files support pylint options these days,
which is preferable to the old .pylintrc which just clutters up the
directory. Prefer setup.cfg for now since flake8 does not support the
newer file (https://gitlab.com/pycqa/flake8/-/issues/428).

setup.cfg

@@ -3,3 +3,15 @@ universal = 1
 
 [flake8]
 max-line-length = 120
+
+[pylint.MESSAGES CONTROL]
+# Katpoint has many short and quirky variable names - leave it for now
+disable = invalid-name
+
+[pylint.TYPECHECK]
+# These Astropy modules generate most of their members
+ignored-modules = astropy.units,astropy.constants
+
+[pylint.FORMAT]
+# The default is 120, but put it in here for visibility if we want to change it
+max-line-length = 120