Validate expression inputs

[drhagen]

We should validate the inputs to the leaves of tensor expressions so that various errors occur earlier rather than later:

1. Validate that the input to Integer is non-negative. Alternatively, we could modify the parser to allow negative numbers.
2. Validate that the input to Float is non-negative finite. Alternatively, we could modify the parser to allow inf and nan and negative numbers.
3. Validate that the name and index names of Tensor follow this regex [A-Za-z][A-Za-z0-9]*