namelist: rearrange names, shortnames, abbreviation & explanation of env pressures

[florisvdh]

The need has been expressed to have shortnames for environmental pressures that are no abbreviation, but a simplification of the current names (which can be more elaborate, hence less appropriate for use in tables & graphs).

The shortname should still contain the numeric code in parentheses, at the end. For example it could be:

Eutrophication through air (3.1)

This would also add an additional column in the return value of read_env_pressures().

At the same time, the existing abbreviations have to be kept (currently stored as shortname in namelist); they are used in read_env_pressures()$ep_abbrev, and included in n2khabmon::read_schemes()$scheme_shortname. One idea might be to move the abbreviations to the shortname of the *_explan items in namelist; to be investigated and consequently implemented in read_env_pressures() and in the generate_textdata projects of both packages.