Re-do group sequential vignette using graphicalMCP

[keaven]

Make a version of the group sequential design vignette that uses the graphicalMCP package instead of gMCPLite.
This should just involve replacing code in the section "Evaluate hypothesis rejection using gMCPLite” and subsequent sections.
Call this vignette “Group sequential design using the graphicalMCP package using hgraph”.
We will still use the hgraph function for plotting.