Renaming decimation->data_fraction and fraction->mu_fraction

[diegodoimo]

I'd like to propose the following change of argument names in the compute_id_2NN and compute_id_scaling_2NN:

• decimation --> data_rate
• fraction --> mu_rate

I know that this is a change that would affect the final user but I think that the previous names are confusing and not expressive at all. Moreover,

• decimation = 0 seems that no data point is decimated;
• decimation = 1 is not immediately clear but suggests a higher degree of data being decimated;

but what we do is exactly the opposite. This is something we should have thought about more in-depth before, but I think that this is a good change to make.

[AldoGl]

Yes, I agree the original names are confusing! However I don't fully understand why the word "rate" is appropriate here. Shouldn't we prefer something like "data_fraction" and "mu_fraction"? Just asking

[diegodoimo]

Yes, I don't know why I proposed 'rate' to mean 'ratio' / 'fraction'. 'fraction' is better.