Update to filesource

Move tprev out of the loop to enable storing the previous time value.
Add some general warning message that an error might have occurred during
reading the time or data values from the data input file.
Allow empty lines in the data input file.
Guard early data values (TIME < time offset) against false reading.
Add some simple examples.

examples/xspice/filesource/filesource_variants.cir

@@ -0,0 +1,49 @@
+simple filesource, time relative or absolute
+
+a1 %vd([out1 0]) filesrc
+
+.model filesrc filesource (file="rect_rel.m" amploffset=[0.1] amplscale=[1]
++ timeoffset=0 timescale=1
++ timerelative=true amplstep=false)
+
+a2 %vd([out2 0]) filesrc2
+
+.model filesrc2 filesource (file="rect_rel.m" amploffset=[0.15] amplscale=[1]
++ timeoffset=10e-6 timescale=1
++ timerelative=true amplstep=false)
+
+a3 %vd([out3 0]) filesrc3
+
+.model filesrc3 filesource (file="rect_rel.m" amploffset=[0.2] amplscale=[0.9]
++ timeoffset=5e-6 timescale=1.2
++ timerelative=true amplstep=false)
+
+a4 %vd([out4 0]) filesrc4
+
+.model filesrc4 filesource (file="rect_rel.m" amploffset=[0.05] amplscale=[1.2]
++ timeoffset=0 timescale=1.2
++ timerelative=true amplstep=true)
+
+
+a5 %vd([out5 0]) filesrc5
+
+.model filesrc5 filesource (file="rect_abs.m" amploffset=[0.05] amplscale=[1.1]
++ timeoffset=5u timescale=1.2
++ timerelative=false amplstep=false)
+
+a6 %v([out6 out7]) filesrc6
+
+.model filesrc6 filesource (file="rect_abs_dual.m" amploffset=[0.05 -0.05] amplscale=[0.9]
++ timeoffset=5u timescale=1.2
++ timerelative=false amplstep=false)
+
+.control
+tran 0.1u 100u
+set xbrushwidth=3
+plot V(out1) V(out2)+2 V(out3)+4 V(out4)+6 V(out5)+8 V(out6)+11 V(out7)+11
+.endc
+
+.end
+
+
+

examples/xspice/filesource/rect_abs.m

@@ -0,0 +1,11 @@
+# simple input, time absolute
+0   0
+10e-6 0
+11e-6 1.8
+21e-6 1.8
+22e-6 0
+32e-6 0
+33e-6 1.8
+43e-6 1.8
+44e-6 0
+1 0

examples/xspice/filesource/rect_abs_dual.m

@@ -0,0 +1,11 @@
+# simple input, time absolut
+0      0     0
+10e-6  0     0
+11e-6  1.8  -0.3
+21e-6  1.8  -0.3
+22e-6  0    -0.6
+32e-6  0    -0.6
+33e-6  1.8  -0.3
+43e-6  1.8  -0.3
+44e-6  0     0
+1 0

examples/xspice/filesource/rect_rel.m

@@ -0,0 +1,11 @@
+# simple input, time relative
+0   0
+10e-6 0
+1e-6 1.8
+10e-6 1.8
+1e-6 0
+10e-6 0
+1e-6 1.8
+10e-6 1.8
+1e-6 0
+1 0