Add metadata information about accessibility

[fchabouis]

closes #138

Add 3 fields to metadatas:

• number of stops (for any location type)
• number of stops with wheelchair_boarding information provided. The tricky part was to check for inheritance, as per the spec, a stop can have the field wheelchair_boarding empty but inherit the information from its parent station.
• the number of trips with wheelchair_accessible information provided

Note : this doesn't count the number of wheelchair accessible stops/trips. We are more interested to know if the information is present or not for the moment.

[fchabouis]

Thank you for the review @antoine-de :purple_heart: