Gaussian peak value helpers

Adde helper property to Gaussian which sets and gets the function value
at the peak. It is simply parameter 'A' scaled by sigma*sqrt(2*Pi).
Renamed Gaussian helper maximum to height

Gave helper property a more intuitive name
Made separate component type

Made a separate component type for a gaussian with 'height' instead of
'A'. This makes the parameter vs. peak maximum independent of sigma, and
thereby makes locking of the parameter more viable. As long as there it
no binning, the height parameter corresponds directly to the peak
maximum, if not, the value is scaled by a linear constant
(signal_axis.scale).
Switched sigma to fwhm
Utility function to get out integral

For Gaussian, the integral is simply 'A', but some conversion needs to
be done for Gaussian2, so include a utility function to get the integral
out.
Bugfix: Forces float for height
DOC: add info about using different matplotlib colormaps for heatmaps
Removed a newline
separated axis slicing functions to be able to get array slices without changing the axis itself
Traitsui view default_button

This should do something, but doesn't seem to...
NEW: early implementation for ROI
ENH: early attempt for implementing variable plot navigators, ROI
WIP
Finished initial suggestion

Finished implementation of ResizebleDraggableRectangle, and tuned the
use of DataAxis.slice.
Renamed new _getitem_tuple, kept old

Realized some things depended on the old implementation of
_getitem_tuple, so kept it, and called the new one _getitem_tuple_all.
Added traits to components and parameters

By adding a view, this would allow components and/or parameters to be
edited by traitsui.
Added component.name/active to traits + fixups

Specified parameter.value better, and fixed bug in when
store_current_value_in_array was called (introduced last commit).
Added bmin/bmax and units as traits as well

Added custom traits type to allow None to still be used, while also
allowing it to be edited through a TextEditor. No other traits should
need to be added at this point from the point of view of a user
modifiable traitsui. Unless I misunderstand the use of ext_bounds and
ext_force_positive.
Added only_current checkbox to ComponentFit

Added a only_current option for the ComponentFit traitsui view.
Implemented it using the CheckListEditor as I could make the label come
on the right of the tickbox, instead of having the label screw up the
layout.
Merge branch 'gaussian_peaks' into work_horse