Allow non-numbers as values to performance variables

[vkarak]

Users would like to use a performance variable to simply include an additional information to the performance log, so the tend to define the following pattern and reference:

self.perf_patterns = {
    'non-numeric-var': sn.extractsingle(...)
}
self.reference = {
    '*': {
        'non-numeric-var': (None, None, None, 'n/a')
    }
}

We could simply avoid the assert_reference()check if the reference value is None or we could even go further by skipping the test if the extracted value is not a number. This would not require to define a special reference for such variables at all.

This request has emerged in #1146 and #1430.