Incorporate validation of CellML files

[jonrkarr]

Can be done once there's a libCellML distribution for Python3.9/Linux (could also be done by compiling from source, but a precompiled distribution is easier).

[jonrkarr]

# read model
parser = libcellml.Parser()
with open(filename, 'r') as file:
    model = parser.parseModel(file.read())
parser.errorCount()
parser.warningCount()

# validate model
validator = libcellml.Validator()
validator.validate(model)
validator.errorCount()
validator.warningCount()

[jonrkarr]

Incorporated validation for CellML 2.0 into 0.1.70. Note, libCellML only supports CellML 2.0 and few, if any, 2.0 models use this version yet.