save-restore: add examples for usage for M70,M72,M73 in nc_files

nc_files/g20sub.ngc

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+;
+
+O<g20sub> sub
+M70 (save in current call context)
+
+g20 (imperial)
+g90 (absolute mode)
+f4  (much faster feed)
+G0 X1.0 Y1.0 (move absolute to 1in/1in)
+
+M71 (restore to caller context)
+O<g20sub> endsub
+
+m2

nc_files/m70m72-demo.ngc

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+; Demonstrate saving and explicitely restoring the global state around
+; a subroutine call using  M70 - save state and M72 - restore state
+;
+; note that the subroutine itself is not aware of the M7x features
+
+O<showstate> sub
+(DEBUG, imperial=#<_imperial> absolute=#<_absolute> feed=#<_feed> rpm=#<_rpm>)
+O<showstate> endsub
+
+O<imperialsub> sub
+;
+g20 (imperial)
+g91 (relative mode)
+F5 (low feed)
+S300 (low rpm)
+;
+(debug, in subroutine, state now:)
+o<showstate> call
+;
+O<imperialsub> endsub
+
+
+; main program
+g21 (metric)
+g90 (absolute)
+f200 (fast speed)
+S2500 (high rpm)
+;
+(debug, in main, state now:)
+o<showstate> call
+;;
+M70 (save caller state in at global level)
+;
+O<imperialsub> call
+;
+M72 (explicitely restore state)
+;
+(debug, back in main, state now:)
+o<showstate> call
+;
+m2

nc_files/m73-demo.ngc

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+; Demonstrate saving the global state within a subroutine
+; using the M73 autorestore feature
+;
+
+O<showstate> sub
+(DEBUG, imperial=#<_imperial> absolute=#<_absolute> feed=#<_feed> rpm=#<_rpm>)
+O<showstate> endsub
+
+
+O<imperialsub> sub
+M73 (save caller state in current call context, restore on return or endsub)
+;
+g20 (imperial)
+g91 (relative mode)
+F5 (low feed)
+S300 (low rpm)
+;
+(debug, in subroutine, state now:)
+o<showstate> call
+(debug, arg=#1)
+o<test_return> if [[#1] GT 0]
+; if called with param #1 > 0, demonstrate restore-on-return
+(debug, executing return)
+o<test_return> return
+o<test_return> endif
+
+; note - no exit M7x code needed - the following endsub or an
+; explicit 'return' will restore caller state
+(debug, executing endsub)
+O<imperialsub> endsub
+
+
+; main program
+g21 (metric)
+g90 (absolute)
+f200 (fast speed)
+S2500 (high rpm)
+;
+(debug, in main after endsub, state now:)
+o<showstate> call
+;
+; show restore-on-endsub:
+o<imperialsub> call [0]
+;
+(debug, back in main after return, state now:)
+o<showstate> call
+; now show restore-on-return:
+o<imperialsub> call [1]
+;
+(debug, back in main, state now:)
+o<showstate> call
+;
+m2

nc_files/nestedcall.ngc

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+
+
+G21 (metric)
+F100 (note slow feed)
+
+G91 (relative mode)
+G1 X10
+
+; this sub switches to G20 (inch) and G90 but protects the caller by M70/M71
+; it also switches to a much higher feed
+O<g20sub> callsub
+
+G1 X10 (this move still relative and in metric units, note slow feed restored)
+m2

nc_files/toplevel.ngc

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+; using M70/M71/M72 at the top level
+; note that the 'Active G-codes' window in Axis isnt properly updated
+; when single stepping
+
+F200
+S1200
+G90 (absolute mode)
+
+M70 (save state)
+
+f50
+s120
+g91 (relative motion)
+g1 x5
+
+M72 (restore without invalidating - multiple use possible)
+(DEBUG, feed and speed back at f200 s1200 plus G90 restored)
+f100
+
+M71 (restore and invalidate)
+(DEBUG, another M71 or M72 here should fail with an error message)
+
+; M71
+
+M2