OTHER: Allow alternate weighting schemes for aspls and lsrpls

[derb12]

Description

Provides a toggle to allow different weighting schemes for aspls and lsrpls (and their pspline and 2D versions). Made the new weighting scheme the default for aspls and kept the original scheme the default for lsrpls. Closes issue #40.

Type of Pull Request

• Bug Fix
• New Feature
• Miscellaneous Changes (refactor, code improvements, etc.)
• Documentation or Example Programs

Pull Request Checklist

• New code and/or documentation is valid for use with the BSD 3-clause license.
• New code is fully documented with docstrings that follow Numpy style,
  if applicable.
• New code follows PEP 8 standards as closely as possible, if applicable.
• Added/updated tests and ensured they pass locally, if applicable.
• Verified that documentation builds locally, if applicable.