Add plate motion model parameters to ITRF init-files.

Parameters for the plate motion models (PMM) for ITRF2008 and ITRF2014
are added to the ITRF2008/2014 init-files. The PMMs allow coordinates
to be moved back and forward in time in plate fixed reference frames
such as GR96 in Greenland which is defined as ITRF94@1996.623.
Transforming an ITRF2014-coordinate to GR96 is done with:

+proj=pipeline +step +init=ITRF2014:NOAM   +t_epoch=1996.623 +t_obs=2017.584
               +step +init=ITRF2014:ITRF94 +t_obs=1996.623

where the first step transforms the coordinate back in time to
ITRF2014@1996.632 by using the ITRF2014 PMM. The second step transforms
the ITRF2014 coordinate to ITRF94.

nad/ITRF2008

@@ -22,3 +22,40 @@
 <ITRF89> +proj=helmert +x=0.0278 +y=0.0386 +z=-0.1012 +s=0.00731 +rz=0.00006 +dx=0.0001 +dy=-0.0005 +dz=-0.0032 +ds=0.00009 +drz=0.00002 +t_epoch=2000.0 +transpose
 
 <ITRF88> +proj=helmert +x=0.0228 +y=0.0026 +z=-0.1252 +s=0.01041 +rz=0.00006 +dx=0.0001 +dy=-0.0005 +dz=-0.0032 +ds=0.00009 +drz=0.00002 +t_epoch=2000.0 +transpose
+
+
+# ITRF2008 Plate Motion Model parameters
+#
+# As described in
+#
+# Altamimi, Z., L. Métivier, and X. Collilieux (2012), ITRF2008 plate motion model,
+# J. Geophys. Res., 117, B07402, doi:10.1029/2011JB008930.
+
+
+<AMUR> +proj=helmert +drx=-0.000190 +dry=-0.000442 +drz=0.000915
+
+<ANTA> +proj=helmert +drx=-0.000252 +dry=-0.000302 +drz=0.000643
+
+<ARAB> +proj=helmert +drx=0.001202 +dry=-0.000054 +drz=0.001485
+
+<AUST> +proj=helmert +drx=0.001504 +dry=0.001172 +drz=0.001228
+
+<CARB> +proj=helmert +drx=0.000049 +dry=-0.001088 +drz=0.000664
+
+<EURA> +proj=helmert +drx=-0.000083 +dry=0.000534 +drz=0.000750
+
+<INDI> +proj=helmert +drx=0.001232 +dry=0.000303 +drz=0.001540
+
+<NAZC> +proj=helmert +drx=-0.000330 +dry=-0.001551 +drz=0.001625
+
+<NOAM> +proj=helmert +drx=0.000035 +dry=-0.000662 +drz=0.0001
+
+<NUBI> +proj=helmert +drx=0.000095 +dry=-0.000598 +drz=0.000723
+
+<PCFC> +proj=helmert +drx=0.000411 +dry=0.001036 +drz=-0.002166
+
+<SOAM> +proj=helmert +drx=-0.000243 +dry=-0.000311 +drz=-0.000154
+
+<SOMA> +proj=helmert +drx=-0.000080 +dry=-0.000745 +drz=0.000897
+
+<SUND> +proj=helmert +drx=0.000047 +dry=-0.001 +drz=0.000975

nad/ITRF2014

@@ -24,3 +24,32 @@
 <ITRF89> +proj=helmert +x=0.0304 +y=0.0355 +z=-0.1308 +s=0.00819 +rz=0.00026 +dx=0.0001 +dy=-0.0005 +dz=-0.0033 +ds=0.00012 +drz=0.00002 +t_epoch=2010.0 +transpose
 
 <ITRF88> +proj=helmert +x=0.0254 +y=-0.0005 +z=-0.1548 +s=0.01129 +rx=0.0001 +rz= +dx=0.00026 +dy=0.0001 +dx=-0.0005 +dz=-0.0033 +ds=0.00012 +drz=0.00002 +t_epoch=2010.0 +transpose
+
+# ITRF2014 Plate Motion Model parameters
+#
+# As described in
+#
+# Z. Altamimi et al, 2017, ITRF2014 plate motion model,
+# doi: 10.1093/gji/ggx136
+
+<ANTA> +proj=helmert +drx=−0.000248 +dry=−0.000324 +drz=0.000675
+
+<ARAB> +proj=helmert +drx=0.001154 +dry=−0.000136 +drz=0.001444
+
+<AUST> +proj=helmert +drx=0.001510 +dry=0.001182 +drz=0.001215
+
+<EURA> +proj=helmert +drx=−0.000085 +dry=−0.000531 +drz=0.000770
+
+<INDI> +proj=helmert +drx=0.001154 +dry=−0.000005 +drz=0.001454
+
+<NAZC> +proj=helmert +drx=−0.000333 +dry=−0.001544 +drz=0.001623
+
+<NOAM> +proj=helmert +drx=0.000024 +dry=-0.000694 +drz=-0.000063
+
+<NUBI> +proj=helmert +drx=0.000099 +dry=−0.000614 +drz=0.000733
+
+<PCFC> +proj=helmert +drx=−0.000409 +dry=0.001047 +drz=-0.002169
+
+<SOAM> +proj=helmert +drx=−0.000270 +dry=−0.000301 +drz=−0.000140
+
+<SOMA> +proj=helmert +drx=−0.000121 +dry=−0.000794 +drz=0.000884