Fix typos and remove some trailing whitespaces

[mmahmoudian]

I fixed some typos and removed trailing whitespaces except those that match the following criteria:

• those spellings that are valid in British on American English. If valid in at least one of them, I didn't change the existing version
• if the whitespace matches ^\s+$, and is outside of a .Rd section

if you want me to adjust all the indentations and spacing, I will be happy to add another commit to this PR for those too.

[dusadrian]

Thanks a lot, merged!