Merging in many dev updates.

* 3D products bit depth now enforced more robustly.

* FITS headers cleaned up, especially for 2D maps.

* Cube noise calculation made more efficient when full NaN planes present.

* Fixed bug in rmsynth3d when supplying I model and not computing FDF

* Adding new tests for secondary/helper tools.

* Converted fixed speed of light values to astropy constant.

setup.py

@@ -10,7 +10,7 @@
 DESCRIPTION = "RM-synthesis, RM-clean and QU-fitting on polarised radio spectra"
 URL = "https://github.com/CIRADA-Tools/RM-Tools"
 REQUIRES_PYTHON = ">=3.7.0"
-VERSION = "1.4.6"
+VERSION = "1.4.7"
 DOWNLOAD_URL = (
     "https://github.com/CIRADA-Tools/RM-Tools/archive/v" + VERSION + ".tar.gz"
 )