changed atomic access for updating the acceleration profile
the stepper interrupt is only halted when necessary and for the shortest
time possible (8% cycle time)
Push additional updates from @jgeisler0303
Added some prelimary notes to new changes.
integrating soft limits
bugfix: uninitiallized curr_block->new_entry_speed_sqr lead to step loss
in some cases
Merge pull request #193 from bungao/soft_limts

integrating soft limits
Minor changes and added notes to soft limits routines.

- Changed up mc_line to accept an array rather than individual x,y,z
coordinates. Makes some of the position data handling more effective,
especially for a 4th-axis later on.

- Changed up some soft limits variable names.
Bug fix to-do note on soft limit checks. Not yet completed.