Change default extension for image plane

[thjsal]

I suggest we change the default local_variables.pyx extension used for the image plane calculation to PDT_U.pyx from the current, which is uniform.pyx. This way most of the other archived extensions should be possible to replicate without re-installing X-PSI (except maybe r-mode and fieldlines extension).

If changing this, we need to modify and re-run those tutorials that use some of the image plane properties (adding more global variables to CustomPhotosphere or as input parameters). But at least those can then be run with the same X-PSI installation.

Besides, looks like the module-generator is already assuming the used extension to be PDT_U.pyx.

[thjsal]

So tutorials that should be then at least updated:

• Modeling without statistics: Add more parameters to the global_variables function of CustomPhotosphere, and remove the comment that X-PSI needs to be installed with a different extension.

• Surface radiation field tools: Add more global parameters to the code that calculates intensities using the global variables approach.