add positive definiteness check for Gaussian proposal

[lijun99]

Added two flags to COV.py

check_positive_definiteness = altar.properties.bool(default=True)
check_positive_definiteness.doc = 'whether to check the positive definiteness of Σ matrix and condition it accordingly'

min_eigenvalue_ratio = altar.properties.float(default=0.001)
min_eigenvalue_ratio.doc = 'the desired minimal eigenvalue of Σ matrix, as a ratio to the max eigenvalue'