API Requirements Fix

Fixed the API to support better metamodel for requirements

for example:
class PropulsionReqs(Requirement):
@classmethod
def define(cls, model):
required = model.parameter("required_vacuum_thrust", unit=kN)
declared = model.parameter("declared_vacuum_thrust", unit=kN)
margin = model.attribute(
"vacuum_thrust_margin", unit=kN, expr=declared - required,
)
model.constraint("thrust_margin_non_negative", expr=margin >= 0 * kN)

    model.requirement(
        "req_vacuum_thrust_capability",
        "The propulsion subsystem shall deliver vacuum thrust no less than "
        "the mission floor (verification by test / analysis).",
    )

See documentation at https://www.thundergraph.ai/open-source/thundergraph-model/docs/user/concepts_requirements.html

v1.0.0

Full Changelog: https://github.com/ThunderGraph/thundergraph-model/commits/v1.0.0