implement straight guide component using numpy

[yxqd]

Before implementing the numba version, make a numpy implementation

• inherits AbstracComponent
• refer to RadialCollimator for basic structure of a component
• need a method to calculate intersection of a neutron ray with a plane
• need a method to calculate reflectivity
• implement the process method
• read McStas guide component implementation for inspiration http://www.mcstas.org/download/components/2.7/optics/Guide.comp
• use numpy vectorization

[mtbc]

@ckendrick: Anything we're missing here?