variable_id validation

[znichollscr]

The motivation

As a validator, any variables with either an underscore or a hyphen in their name should fail. These can cause many kinds of downstream software to trip up, so users should just use camel case instead if they need.

The proposed solution

Add validation of variable names (hence variable_id).

Alternatives

Additional context

Original discussion here: PCMDI/input4MIPs_CVs#131 (comment)