ExternalData: Add option to recursively match under directories

Extend the ``DATA{Dir/,...}`` syntax with a new ``RECURSE:`` option
to enable recursive matching of associated files.  This will allow
an entire directory tree of data to be referenced at once.

Tests/Module/ExternalData/CMakeLists.txt

@@ -44,6 +44,7 @@ ExternalData_Add_Test(Data1
     -D Paired=DATA{PairedA.dat,PairedB.dat}
     -D Meta=DATA{MetaTop.dat,REGEX:Meta[ABC].dat}
     -D Directory=DATA{Directory/,A.dat,REGEX:[BC].dat}
+    -D DirRecurse=DATA{DirRecurse/,RECURSE:,A.dat,REGEX:[BC].dat}
     -D "Semicolons=DATA{Data.dat}\\;DATA{Data.dat}"
     -P ${CMAKE_CURRENT_SOURCE_DIR}/Data1Check.cmake
   )

Tests/Module/ExternalData/Data1Check.cmake

@@ -90,6 +90,12 @@ foreach(n A B C)
     message(SEND_ERROR "Input file:\n  ${file}\ndoes not exist!")
   endif()
 endforeach()
+foreach(n A Sub1/A Sub2/Dir/A B Sub1/B Sub2/Dir/B C Sub1/C Sub2/Dir/C)
+  set(file "${DirRecurse}/${n}.dat")
+  if(NOT EXISTS "${file}")
+    message(SEND_ERROR "Input file:\n  ${file}\ndoes not exist!")
+  endif()
+endforeach()
 list(LENGTH Semicolons len)
 if("${len}" EQUAL 2)
   foreach(file ${Semicolons})

Tests/Module/ExternalData/DirRecurse/A.dat.md5

@@ -0,0 +1 @@
+9d980b06c2f0fec3d4872d68175b9822

Tests/Module/ExternalData/DirRecurse/B.dat.md5

@@ -0,0 +1 @@
+8f4add4581551facf27237e6577fd662

Tests/Module/ExternalData/DirRecurse/C.dat.md5

@@ -0,0 +1 @@
+c1030719c95f3435d8abc39c0d442946

Tests/Module/ExternalData/DirRecurse/Sub1/A.dat.md5

@@ -0,0 +1 @@
+9d980b06c2f0fec3d4872d68175b9822

Tests/Module/ExternalData/DirRecurse/Sub1/B.dat.md5

@@ -0,0 +1 @@
+8f4add4581551facf27237e6577fd662

Tests/Module/ExternalData/DirRecurse/Sub1/C.dat.md5

@@ -0,0 +1 @@
+c1030719c95f3435d8abc39c0d442946

Tests/Module/ExternalData/DirRecurse/Sub2/Dir/A.dat.md5

@@ -0,0 +1 @@
+9d980b06c2f0fec3d4872d68175b9822

Tests/Module/ExternalData/DirRecurse/Sub2/Dir/B.dat.md5

@@ -0,0 +1 @@
+8f4add4581551facf27237e6577fd662

Tests/Module/ExternalData/DirRecurse/Sub2/Dir/C.dat.md5

@@ -0,0 +1 @@
+c1030719c95f3435d8abc39c0d442946