Clean up object size usage

[martinkilbinger]

Various estimators of object size are used throughout the code:

• fwhm
• sigma (e.g. from HSM moment measurement)
• T = I_xx + I_yy (sum of second-order moments), also written as R^2, T approx 2 sigma^2

Units can be in pixels or arcsec/arcmin.

These should be unified and made clear.

Two functions to transform between some of those exist in sp_validation/galaxy.py, we should use and extend those.

In the code, replace hard-coded numbers 1.17741, 2.355.

[sfarrens]

Tagging @lbaumo