Added 'assert' pseudo function so I can run some tests of the script code

git-svn-id: svn://svn.code.sf.net/p/magicseteditor/code/trunk@988 0fc631ac-6414-0410-93d0-97cfa31319b6

doc/function/assert.txt

@@ -0,0 +1,16 @@
+Function: assert
+
+--Usage--
+> assert(condition)
+
+Check that the condition is @true@. Shows a warning message if it is not.
+
+Note: @assert@ is a special built-in keyword, so that the error message can include the condition.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:boolean]]	Condition to check.
+
+--Examples--
+> assert(1 + 1 == 2) == nil  # nothing happens
+> assert(1 * 1 == 2) == nil  # An error message is shown