Merge pull request #3 from ebarre/master

Update README with extra note
Added new field base class TField for all field calculation methods to facilitate implementation of new ones
Updated welcome message
Merge branch 'master' of https://github.com/wschreyer/PENTrack
Added field.h
Switched to NR random number generator and improved random seed generation
Testing sqrt distribution
Bugfix: Wrong value was assigned to TParticle::Estart
Update README.md
Adding STL files from martin's newsource_stl

Merge remote-tracking branch 'upstream/master'

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changed the geometry file and also updated makefile with recommendation
from Wolfgang (-lrt)
Merge remote-tracking branch 'upstream/master'
Added in dafault material and ignored id into geometry
geometry from martin's newsource_stl