Issue 3553/1: get_default_interpreter should create contexts with missing context_id

Issue 3553/1: get_default_interpreter should create contexts with missing context_id

At the current point of time, an existing bottom context is returned
even when there is a mismatch in ids.  For example, when writing

\context Voice = "one" <<
  { \voiceOne g' g' g' g' }
  \context Voice = "two" { \voiceTwo c' c' c' c' }
>>

creates two voices, while

\context Bottom = "one" <<
  { \voiceOne g' g' g' g' }
  \context Bottom = "two" { \voiceTwo c' c' c' c' }
>>

only uses a single Voice called "one".  This is arguably wrong.