Mandatory logging

[ikbuibui]

We have a bitmask for various logging levels in PIConGPU. The default logging level is PHYSICS = 1. For places where we must warn the user irrespective of the set logging level, there is no obvious solution on how to do this.
A few options are

• Using CRITICAL and enabling this by default - This might be misleading since I think "critical" implies errors
• Add a new MANDATORY level bit which is enabled by default. This can be something like 1ULL<<7 or also 1ULL<<63
• Custom log levels per code section - Similar to what radiation does, but IMO this adds complexity without much benefit
• Having a always-true bitmask and call this MANDATORY - We can use a value like ~0ULL (all bits set) so any bitwise & operation returns true. This will require some changes to the logging output logic as it will otherwise say that logs made by MANDATORY are actually made by the level enabled by default.

[ikbuibui]

@psychocoderHPC @chillenzer Any opinions?