removed g_ncid as the ascociated bug was fixed in c library 4.4.1

[cptspacemanspiff]

Fixes #32 #30
Hi, So I have been digging into the global variable issue, it appears it was fixed in the C library in 2016.

specifically It seems like this was fixed in the netcdf-c 4.4.1 library:
Unidata/netcdf-c#240

If my understanding is correct, as it stands anyone using netcdf4 and defining custom types cannot have more than one file open at the same time. It also forces the custom type definition to be in the root group for netcdf4, rather than the parent group as one would expect.

I think the correct solution is to get rid of the global variable g_ncid and replace it with the groupID when calling nc_inq_type, though I am not sure what version compatibility you are targeting between this and netcdf-c.

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