Refactor generate proof function when a group has only one member and tree depth 0

[vplasencia]

Description

When there is 1 member in the group and you want to generate a proof without passing the tree depth, there will be an error because tree depth 0 is not supported to generate a proof and one member is tree depth 0. It's true that in a group with 1 member there is no anonymity but for testing purposes it's better to also support tree depth 0 when generating a proof for groups with 1 member without passing the tree depth parameter.

Tasks:

• Update the generate proof function to fix this one member issue when the tree depth is not passed as a parameter. #812
• Update boilerplate to allow people to generate proofs in groups with 1 member boilerplate#63

More context: #804