Improve printing and allow Entry to consist of different AccountTypes.

Generlaize so methods are not tied to Account, but use abstract AccountType instead

• cleanup
• Remove iscontra
• Maye keyword arguments
• TypeVars
• Identifier
• Ledger
• A -> Type{A}
• Add AccountWrapper

• Add convenient constructor and swap name <-> number order.
• Add convert method for AccountNumber
• Add indexes to AccountGroup
• Add getindex for AccountGroup
• Rename add_account! -> add_account

Update

• Change AccountGroup parameter from <: Position to <: AccountType
• Add accounttype method
• Improve printing
• Remove AccountNode
• account -> ledgeraccount
• AccountNumber()
• add_account!

Update

• Rename Account -> LedgerAccount
• Rename AccountInfo -> Account
• Account constructor can take a ledger keyword
• Add some default constructors
• Update for changes in dependencies, e.g. USD is a position type

• Use the new convenience position types
• Remove import (use explicit module name instead)