[ENH] Benchmarks for causal discovery algorithms on Linear Gaussian Data

[ankurankan]

pgmpy currently supports a few causal discovery algorithms:

1. PC
2. GES
3. Hill-Climb Search
4. TreeSearch

Add a benchmark suite using linear Gaussian data to test how well these methods can recover the true graph.

[Vanshitaaa20]

@ankurankan assign this to me..