Review `ParserBase` member access modifiers to ensure they match those in `hapi`.

[milkshakeuk]

Describe the Issue
It's not clear that the access modifiers in the ParserBase and any class that inherits from it are using the correct access modifiers for all its members.

I suspect there are some public members which shouldn't be public, I suspect that during the original porting process looking at the default access modifiers for java wasn't compared to the default access modifiers for C#, since these are slightly different.

This issue is just to review those modifiers to make sure they match - it may mean a change in contract which may mean a major nHapi revision.