Add available precharge and rollout info

[philipnbbc]

This PR adds these optional properties to the mxf2raw clip section:

• max_available_precharge
  • Is the maximum number of available edit units (expressed as a negative number) before clip position 0 of all tracks in the clip
  • A larger (negative) max_available_precharge compared with max_precharge means there are more edit units available than strictly necessary to produce the output
• max_available_rollout
  • Is the maximum number of available edit units (expressed as a positive number) after clip position clip duration - 1 of all tracks in the clip
  • A larger max_available_rollout compared with max_rollout means there are more edit units available than strictly necessary to produce the output

This PR also adds these optional properties to the mxf2raw track section:

• available_precharge
  • Is the number of available edit units (expressed as a negative number) before track position 0
  • A larger (negative) available_precharge compared with precharge means there are more edit units available than strictly necessary to produce the output
• available_rollout
  • Is the number of available edit units (expressed as a positive number) after track position track duration - 1
  • A larger available_rollout compared with rollout means there are more edit units available than strictly necessary to produce the output

This example file from #23 produces this output:

...
Clip
  edit_rate       : 25/1
  duration        : 00:01:59:02 (count='2977')
  max_precharge   : -2
  max_available_precharge : -14
...
    Track #0:
      essence_kind    : Picture
      essence_type    : MPEG_2_Long_GOP_422P_HL_1080i
      ec_label        : urn:smpte:ul:060e2b34.04010102.0d010301.02046001
      edit_rate       : 25/1
      duration        : 00:01:59:02 (count='2977')
      precharge       : -2
      available_precharge : -14
...

2 edit units are necessary to produce the output (the max_precharge and precharge). 14 edit units are available (the max_available_precharge and available_precharge).