Add required option to EasyConfig.get_cuda_cc_template_value method

[Flamefire]

In many places it isn't an error to have no CUDA compute capabilities. Having a parameter to handle this case allows to avoid having to check the EC parameter and the build option in every case causing potential inconsistencies.

With this we can do replacements like:

-        # list of CUDA compute capabilities to use can be specifed in two ways (where (2) overrules (1)):
-        # (1) in the easyconfig file, via the custom cuda_compute_capabilities;
-        # (2) in the EasyBuild configuration, via --cuda-compute-capabilities configuration option;
-        cuda_cc_list = build_option('cuda_compute_capabilities') or self.cfg['cuda_compute_capabilities'] or []
+        cuda_cc_list = self.cfg.get_cuda_cc_template_value("cuda_cc_space_sep", required=False).split()

[jfgrimm]

lgtm