Issue673 si units rc

[adriendaros]

Motivations:

Use an adequat format to describe the units of the physical quantity involved in the file tofu/spectro/_rockingcurve_def.py

Results:

Choice to use the library scipy.constants, in a first time, to allocate the unit in a String format. The next step will be to change into an astropy.units format, more convenient to use (not String format)

Issues:

Fixes, in devel, issue #673

Examples:

[adriendaros]

Issue fixed in ToFuProject/datastock