Fix Get Nested Prerequisites Method

[patrick-5546]

Uses recursion to get all the prerequisites required for a course, stored in the preq field of Course

Relevant code:

• ubcexplorer.py

Example

Consider the tree:

What should be stored:

{child1: {}, child2: {child21: {}, child22: {}}, child3: {}}

Real Example

Immediate prerequisites of MATH 256 (under key preq in dictionary that course_info() returns):

['MATH 101', 'MATH 103', 'MATH 105', 'MATH 121', 'SCIE 001', 'MATH 152', 'MATH 221', 'MATH 223']

Prerequisites of MATH 223 include MATH 121. Prerequisites of MATH 121 include MATH 120 and MATH 100.

What course_info_with_prereq_tree("MATH", "256") should return:

{..., MATH 223: {MATH 121: {MATH 120: {}, MATH 100: {}, ...}, ...}}