added .xlsx output

Added ability to download results as .xlsx spreadsheets. Added notice to header comment, license, and readme, that two functions derive from stack overflow code and are under the CC BY-SA 3.0 license. Reduced the number of global variables. Spreadsheet generation now occurs after download link is clicked.

LICENSE.txt

@@ -1,13 +1,13 @@
 
-high_voltage_divider.html contains the function download_csv(),
-which comes from a Stack Overflow post
+high_voltage_divider.html contains the functions download_csv() and
+download_xlsx(), both of which come from a Stack Overflow post
 https://stackoverflow.com/a/33542499
 by user Ludovic Feltz (on 2015-11-5), edited by user Smi (on 2016-08-12).
 I retrieved this code on 2018-03-31 and changed it slightly.
 Stack Overflow code is distributed under the CC BY-SA 3.0 license:
 https://creativecommons.org/licenses/by-sa/3.0/
 The remainder of high_voltage_divider.html is under the CC0 1.0 Universal
-Public Domain Dedication (reproduced below), as is the entirety of 
+Public Domain Dedication (reproduced below), as is the entirety of
 high_voltage_divider.svg.
 
 Creative Commons Legal Code
@@ -130,4 +130,4 @@ express Statement of Purpose.
     Work.
  d. Affirmer understands and acknowledges that Creative Commons is not a
     party to this document and has no duty or obligation with respect to
-    this CC0 or use of the Work.
+    this CC0 or use of the Work.

README.md

@@ -27,9 +27,9 @@ Ben Tesch - [slugrustle](https://github.com/slugrustle)
 
 ## License
 
-high\_voltage\_divider.html contains the function download_csv()
-which is modified from [this Stack Overflow post](https://stackoverflow.com/a/33542499)
-and is therefore distributed under the 
+high\_voltage\_divider.html contains the functions download_csv() and download_xlsx(),
+which are modified from [this Stack Overflow post](https://stackoverflow.com/a/33542499)
+and are therefore distributed under the 
 [CC BY-SA 3.0 license](https://creativecommons.org/licenses/by-sa/3.0/).
 
 The remainder of the high\_voltage\_divider project is distributed under the