taking into account point vs mean time cell_methods when selecting resample

[paolap]

In mop_setup.py function find_matches() if an exact match can't be found currently create a list of near matches with same cmor_var name and realm but different frequency, to see if resample can be applied to obtain desired frequency.
This means that if as an example I'm looking for "ta" which implies "time: mean".
"ta_Pt" (time:point) is currently excluded from near-matches.
It should be included instead as once we resample this become a mean anyway!

This impact mostly input data from higher frequency see also #23