Implement proper x,y,(z?) co-ordinate tracking for creature positions

[BenjiAU]

In the original percy vs grog script, Grog would just run towards Percy at max speed, and would always start at a higher position value than Percy; the movement functions were written with this into account.

This means:

• melee attackers will only move if they have a higher 'position' than the target
• melee attackers make melee attacks from range if their position is 'lower' than the target

This is all borked up. Needs to be replaced with a proper co-ordinate system;

• define x,y co-ordinates for creature
• define minimum and maximum x,y values for a 'map'
• initialise_position should define both x,y co-ordinates for target
• each co-ordinate should correspond to a 5x5 grid to make determining spell/creature effects easier in the future
• get_distance and other distance comparison functions should compute the shortest number of 5x5 grids between two points (allow diagonal movement/attack)

Also needs to have flexibility for a z co-ordinate in the future (to allow for flying/climbing enemies)

[BenjiAU]

Preliminary x/y co-ordinate system added 17/05/18 - need to get better at assigning commits to issues