Environment Variables for controllable target generation

[mhl787156]

This PR adds a number of environment variables to control target generation explicitly:

Spawning Environment Variable Options

The primary options are given by the following:

Name	Default Value	Description
SPAWN_TARGET_NUM_TARGETS	2	Number of Yellow Targets being spawned
SPAWN_TARGET_NUM_HOTSPOTS	5	Number of Red Targets being spawned
SPAWN_TARGET_HOTSPOT_RADIUS	3	Radius of Red circular hotspots
SPAWN_TARGET_TARGET_LENGTH	20	Length of yellow target
SPAWN_TARGET_TARGET_WIDTH	5	Width of yellow target
SPAWN_TARGET_RANDOM_SEED	None	Random seed for random generation, used for deterministic arrangements.
SPAWN_TARGET_FILE_PATH	''	File Path to fixed spawn, see example file i 'fenswood/target/default_target.json'

Change Size and Location of Volcano Annulus:

Name	Default Value	Description
SPAWN_TARGET_ANNULUS_RADIUS	40	Radius of Volcano
SPAWN_TARGET_ANNULUS_LOC_X	-195	Location Relative to Spawn/Takeoff Location
SPAWN_TARGET_ANNULUS_LOC_Y	-163	Location Relative to Spawn/Takeoff Location
SPAWN_TARGET_ANNULUS_LOC_Z	0.1	Location Relative to Spawn/Takeoff Location

Change Random Spawn Parameters Name	Default Value	Description
SPAWN_TARGET_GEN_TARGET_LOC_ANGLE_MIN	-pi/2	Target location generationg minimum angle
SPAWN_TARGET_GEN_TARGET_LOC_ANGLE_MAX	pi/2	Target location generation maximum angle
SPAWN_TARGET_GEN_HOTSPOT_ANGLE_VARIANCE	0.3	Hotspot sampling normal variance for angle variation relative to target centers
SPAWN_TARGET_GEN_HOTSPOT_RADIUS_VARIANCE	0.5	Hotspot sampling normal variance for radial variation relative to target centers
SPAWN_TARGET_GEN_BETWEEN_TARGET_LOC_ANGLE_MIN	pi/3	Minimal angle targets
SPAWN_TARGET_GEN_BETWEEN_TARGET_LOC_ANGLE_MAX	4	Maximal distance between hotspots

Note that if minimal angle and maximal distance are set incorrectly, it may cause an indefinite sampling as valid samples cannot be generated.