When VDF attributes are missing, please use default alpha and beta values

[asu-trans-ai-lab]

In the current version, VDF attributes are required in link.csv. They should be optional with default values used in both python and DTALite kernel versions. the capacity field in link.csv will be also used first if VDF_cap1 does not have values.

[jdlph]

Thank you for pointing it out! It has been addressed in dev/v0.6.1 under commit cdf4614 which will be merged into the main branch.