Add transaction method for multi-stage updates

[hdoupe]

• Rollback to previous state after errors/exceptions
• Optionally defer schema-level validation until the update is complete
• Change is_deserialized arg to deserialized

import paramtools

class Params(paramtools.Parameters):
    defaults = {
        "min_param": {
            "title": "Min param",
            "description": "Must be less than 'max_param'",
            "type": "int",
            "value": 2,
            "validators": {
                "range": {"max": "max_param"}
            }
        },
        "max_param": {
            "title": "Max param",
            "type": "int",
            "value": 3
        }
    }

params = Params()
with params.transaction():
    params.adjust({"min_param": 4})
    params.adjust({"max_param": 5})

params.sel["min_param"]

# Values([
#   {'value': 4},
# ])

params.sel["max_param"]

# Values([
#   {'value': 5},
# ])

[hdoupe]

Merging #123. There may be some things that ParamTools can do down the road to optimize which values are validated after a transaction, but for now, the lack of optimization here doesn't appear to have a significant performance impact.