No need to explicitly drop traitblock in behavior block

If an applicable is trait is added/present, the behavior input is removed,
and the trait is silently applied to the behavior block.
No trait block panel (yellow panel) is shown either.
For mutation slider, no check is made for presence of reproduce block

For activity 4, mutation slider appears on the interface panel irrespective
of whether reproduce behavior block is used. If reproduce block is not used,
the slider does nothing (no reference of it in the NetLogo code).
Added "Make Distribution" button.

Button still needs to be sized properly.
Misc procedures related to make-distribution no longer throw errors

The procedure code for misc procedure is populated only if a trait is
defined. If no trait is defined, the procedure code (body of the procedure)
is blank, hence no error about undefined species or traits is thrown.

Also, the make distribution button doesn't appear until at least one
trait is defined.
Increased NetLogo world size and moved plots to right

New world dimensions:
min_pxcor=-25, max_pxcor=25, min_pycor=-20, max_pycor= 20, patch_size=13

New plot X offset: 900

Wondering if these (and other such) alues should be part of XML.
Added some interface layout constants to XML

NetLogo world size and plots related constants can be initialized from XML.
This will help tweak the layout on the fly for different screen resolutions.
Added draw button

Tweaked existing code for draw button to enable it. The library has an XML tag
<drawFeature> to enable/disable draw.