Changed name of module 'plot_builtin_wave' to 'plot_source_wave'. Add functionality to allow plotting (with FFT) in Jupyter notebooks of user-defined waveforms.

tools/Jupyter notebooks/example_Ascan_2D.ipynb

@@ -98,7 +98,7 @@
    "source": [
     "%matplotlib inline\n",
     "from gprMax.waveforms import Waveform\n",
-    "from tools.plot_builtin_wave import check_timewindow, mpl_plot\n",
+    "from tools.plot_source_wave import check_timewindow, mpl_plot\n",
     "\n",
     "w = Waveform()\n",
     "w.type = 'ricker'\n",