Simplify the use of airGR in airGRiwrm

The current usage needs the following preamble:

library(airGR)
library(airGRiwrm)

In order to have in the scope the airGR functions always needed by the user (such as ErrorCrit...).

Moreover running airGR and airGRiwrm in the code often leads to load and detach airGR and airGRiwrm libraries which is a real pain (For example see: #43 (comment 41579)).

I think that a "Depends" dependency on airGR instead of "Imports" would solve this problem. As it is said here:

Depends is used to indicate dependency on a particular version of R, and on packages that are to be loaded (with library()) whenever your package is loaded. If you expect that users would want to load that other package whenever they loaded yours, then you should include the package name here. But this is now relatively rare. (I think the namespaces for these packages should also be imported, with @import.)

I checked on a small example with a 'toto' library with a "Depends: airGR" and airGR function are directly accessible.

In that way a simple library(airGRiwrm) would give complete access to airGR functions. It remains to check if we can completely run airGRiwrm and airGR functions by only loading airGRiwrm.