[enh] Frequency generated metadata could be templated with jinja

[bzah]

Originally posted by @bzah in #223 (comment)

For now we have a EN_FREQ_MAPPING dictionary that contains many kind of information in order to turn a Frequency object into a readable metadata string.

It works, but it's not optimal as we don't handle well plurals, plus the mapping of magnitude is done statically (within a python intrepretade string) and it makes it hard to render frequencies for different languages.
Additionally this would help to gave a consistent way to interpret python objects into metadata all over icclim.