Add muon index cut

Add INDEX cut for MU objects, mapped to new scale:

<scale>
  <object>MU</object>
  <type>INDEX</type>
  <minimum>+0.0000000000000000E+00</minimum>
  <maximum>+1.2700000000000000E+02</maximum>
  <step>+1.0000000000000000E+00</step>
  <n_bits>7</n_bits>
  <bin ...>
</scale>

See also https://its.cern.ch/jira/browse/CMSLITDPG-1075