create `make_vector_mask`

[geryan]

• to allow creation of global vector masks
• becomes sub-function of make_africa_mask
• NB make_africa_mask already does this but don't want to remove the function and it has hard-coded extents for raster layers that match Africa
• E.g. make_africa_mask(type = "vector", countries = "SGP") works but is a but silly given function name

[geryan]

Creating make_mask in branch of that name with aliases for make_africa_mask and make_vector_mask

• make_mask works but make_vector_mask and make_africa_mask no longer do.

  • is this a problem with the way the functions are aliased/stored in the same script make_mask.R?

• problem with hard coded extent for rasters in `make_mask(...,type = "raster",...).

  • Maybe hard code for make_africa_mask but otherwise get from vector?

[geryan]

see #69

[geryan]

resolved by #72

[geryan]

NB hard coding of extent is gone, extent is simply gotten from vector mask. This could be breaking

• problem with hard coded extent for rasters in `make_mask(...,type = "raster",...).

  • Maybe hard code for make_africa_mask but otherwise get from vector?