obit: Swap plplot dependency for pgplot

The standard version of plplot on Debian / Ubuntu / etc is the
double-precision one resulting in a library named libplplotd instead of
libplplot. This is also the version installed by this tap.

Unfortunately Obit requires the single-precision version, and these
versions cannot co-exist. We therefore switch to the more convenient
pgplot library (Obit can use either of these two libraries) which works
in its standard incarnation. Remove the patch to plplot.m4 as well,
which means that libplplotd will not be picked up even if installed.
obit: Add testContourPlot test

Install the first test script and corresponding data to the share
directory. The test copies the data to a temp directory, runs the test
and verifies that the output is present before deleting it again.
obit: Reduce verbosity of install action

Reduce the verbosity of installation procedure, while also highlighting
the fact that three separate packages are being built. Display the Obit
version number more prominently. Only keep the configure/make/make
install pattern for each package. The use of safe_system ensures that
warnings and errors still get reported, unlike quiet_system.
obit: Use FileUtils instead of system calls

Since the FileUtils commands are included in the Formula class, these
system commands are cleaner and simpler to use than full-blown system
calls (and still remain quiet).
obit: Switch to standard environment

We need to find the LaTeX executables outside the usual superenv path
and we want to keep our debug symbols. The simplest solution is to
revert to the standard build environment.