Add warning for negative distances during rate-adjustment

[Cecilia-Sensalari]

During rate-adjustment, ksrates prints a warning if there are negative values for Ks distances, stating that they are artefacts.
More details below.

The rate-adjusted ortholog peak (K_AB) is derived through the following mathematical formulas, using three estimated ortholog distribution peaks (A: focal species, B: sister species, C: outgroup):

The numerator in (2) should always in principle be a positive number, as it is a measure of genetic distance. However, poor peak estimate for the three AC, AB and BC ortholog distributions might produce a negative sum, and therefore a negative K_OA value. The negative sign is then maintained also in (4).
Poor peak estimate is often due to the very old divergence age between the involved species or by bad quality sequence data.