MNT: remove development modules (#165)

Removes the `covariance` and `plot` modules. While these are useful for
development, they are not (yet?) at the level of production quality.

Closes: #156
Closes: #159
Reviewed-by: Krishna Naidoo <krishna.naidoo.11@ucl.ac.uk>

Author: ntessore

.commitlint.rules.js

@@ -9,12 +9,10 @@ module.exports = {
         "catalog",
         "cli",
         "core",
-        "covariance",
         "fields",
         "healpy",
         "io",
         "mapper",
-        "plot",
         "progress",
         "twopoint",
       ],