Consider renaming namespace ID to namespace

## Context

This library is used in celestia-app. After [celestia-app/adr-014](https://github.com/celestiaorg/celestia-app/blob/fc83b04c3a5638ac8d415770e38a4046b84fa128/docs/architecture/adr-014-versioned-namespaces.md) and [celestia-app/adr-015](https://github.com/celestiaorg/celestia-app/blob/fc83b04c3a5638ac8d415770e38a4046b84fa128/docs/architecture/adr-015-namespace-id-size.md) were introduced, the namespaced data that gets pushed to an NMT consists of: `namespace version (1 byte) | namespace ID (28 bytes) = namespace (29 bytes)`.

## Problem

It is a bit confusing that this repo refers to them as namespace IDs when celestia-app refers to them as namespaces.

## Proposal

Rename namespace ID to namespace in this repo

Read the namespace terminology in celestia-app/docs/architecture/adr-014-versioned-namespaces.md and adr-015-namespace-id-size.md, then search this repository for references to “namespace ID.” The work is done when the repository consistently uses “namespace” without leaving tests or documentation inconsistent.

Written by the indexing model from the issue text.