planar_graphs generator: support minimum_connectivity=4

[Ordoviz]

plantri supports -pc4 since version 5.0 released on October 2, 2016 but the planar_graphs() generator incorrectly claimed that minimum_connectivity=4 was not supported. This pull request rectifies that.

I also tried adding support for a more broad range of options to the triangulations() generator but I found it difficult to deal with the complex requirements between the various parameters. According to its README, plantri supports the following options, but not all combinations are allowed:

Planar triangulations
    connectivity 1, 2, 3, 4 or 5
    minimum degree 3, 4 or 5
    eulerian with connectivity 3 or 4
Planar quadrangulations
    arbitrary simple
    3-connected
    minimum degree 3 and simple
    3-connected, no non-facial 4-cycles
    arbitrary
Planar simple graphs
    connectivity 1, 2, 3 or 4
    minimum degree 1, 2, 3, 4 or 5
Planar simple bipartite graphs
    connectivity 1, 2 or 3
    minimum degree 1, 2, or 3
Triangulations of a disk
    connectivity 2 or 3
    minimum degree 2 or 3
Planar cubic graphs
    connectivity 1, 2, or 3
    connectivity 1, 2, or 3
    girth 3, 4 or 5
    cyclic connectivity 3, 4 or 5
    bipartite with cyclic connectivity 3 or 4
Planar quartic graphs
    3-connected simple
    4-edge-connected simple
    4-edge-connected
    3-connected, 6-cyclically-edge-connected
    arbitrary

[Ordoviz]

I just reformatted the doctests to fit into 80 columns.

[cxzhong]

Thanks. LGTM