Refs #322: Mostly tidying things up.
Refs #322: Mostly tidying things up.

* Updated exceptions to use preferred Python syntax and removed some redundant 
  TypeError traps.

* Replaced spaces in NXobject names by underscores.

* Removed check for the dimension products being negative due to overflows.

* In log plots, the minimum value of 0.1 is only used for integer data, where it
  is a reasonable replacement for values of 0. In floating point data, the default
  matplotlib behaviour is restored (but can be overridden by using the zmin keyword
  argument).  



git-svn-id: 1001b6e0dd469eaa302a741812f7f5d223cf35f7@1819 ff5d1e40-2be0-497f-93bd-dc18237bd3c7

chrisemblhh/nexus,chrisemblhh/nexus,nexusformat/code,nexusformat/code,nexusformat/code,chrisemblhh/nexus,chrisemblhh/nexus,nexusformat/code,chrisemblhh/nexus,nexusformat/code