Wrap GMT_Inquire_VirtualFile to get the family of virtualfiles

[seisman]

Description of proposed changes

Upstream documentation: http://docs.generic-mapping-tools.org/dev/devdocs/api.html#gmt-inquire-virtualfile

The GMT_Inquire_VirtualFile API function takes the name of a virtual file as input, and returns the integer of the family.

This PR wraps up the API function. Tests are also added.

Address the request in #3115 (comment).

After this PR is merged, we will refactor

def virtualfile_to_grid(self, vfname, outgrid):
    if outgrid is not None:
        return None
    return self.read_virtualfile(vfname, kind="grid").contents.to_dataarray()

to something like

def virtualfile_to_raster(self, vfname, outgrid, kind="grid"):
    if outgrid is not None:
        return None
    if kind is None: 
        family = self.inquire_virtualfile(vfname)
        kind = {
            self["GMT_IS_GRID"]: "grid",
            self["GMT_IS_IMAGE"]: "image",
            self["GMT_IS_CUBE"]: "cube",
       }[family]
    return self.read_virtualfile(vfname, kind=kind).contents.to_dataarray()