Adopt _PACKAGE sentinel for package documentation

[kylebaron]

This PR adopts the "_PACKAGE" approach. See https://github.com/r-lib/roxygen2/issues/1491

Dear maintainer,

You have file 'mrgsolve/man/mrgsolve_package.Rd' with \docType{package}, likely intended as a package overview help file, but without the appropriate PKGNAME-package \alias as per "Documenting packages" in R-exts.

This seems to be the consequence of the breaking change

Using @docType package no longer automatically adds a -package alias. Instead document _PACKAGE to get all the defaults for package documentation.

in roxygen2 7.0.0 (2019-11-12) having gone unnoticed, see https://github.com/r-lib/roxygen2/issues/1491.

As explained in the issue, to get the desired PKGNAME-package \alias back, you should either change to the new approach and document the new special sentinel

"_PACKAGE"

or manually add

@aliases mrgsolve-package

if remaining with the old approach.

Please fix in your master sources as appropriate, and submit a fixed version of your package within the next few months.

[kylebaron]

Thanks, @kyleam