Add User Tutorial

[pp-mo]

Part of "proper" docs, as in #30

topics so far...

• use of threadlocks (explain scope of need)
• array values are attributes Document array nature of Attribute values #67
• netcdf features : supported + unsupported Document netcdf feature support #28
• (better) examples of modification
  • basic of NameMaps
  • nature of core objects (NcData, NcDimension, NcVariable, NcAttribute)
  • attribute creation (cf A neater way to add attrs #71)
  • rename
  • copying and equality testing (difference description)
• basic ops : load/save/copy
• list of minor utils : save-checks, dataset-difference, dim-slicing ...

[pp-mo]

Done-ish #109