protecting names in schema language

[bendichter]

Right now the schema language document does not indicate that there are any rules for name, default_name, neurodata_def, or neurodata_inc. I think we should add this pattern:
"^[A-Za-z_][A-Za-z0-9_]*$"
which means each name must start with a letter can can contain letters, numbers, and underscores, but not spaces, periods, or other punctuation.

The motivations are:

1. matnwb has issues when names contain spaces.
2. pynwb has issues when names contain slashes.
3. This allows us to more easily create classes with names that match the schema, which is not strictly necessary but is a good idea.

The entire core schema currently follows this rule.