Minor textual fixes

[burggraaff]

Some very minor textual fixes I made while reading through the documentation.

• Fixed typos, capitalisation, punctuation in docs.
• Fixed typos, capitalisation, punctuation in docstrings and comments.
• Fixed typos in Exception/Error messages.
• Removed a reference to a CABOFileReader.get_header() method that doesn't exist.
• Updated reference to new NASA Power website.
• Updated examples in documentation to Python 3's print() function.
• Whitespace improvements (done automatically by my IDE).

[burggraaff]

Incidentally and unrelated to the PR, while reading doc/index.rst, I saw that it mentions PCSE running in Python 2.7+ and 3.2+. Is this still accurate? For example, some modules use f-strings, which were introduced in Python 3.6. Do you intend to maintain such extensive backwards compatibility in the future, or is this simply an outdated description?

[ajwdewit]

Dear Burggraaf, this merge runs into conflicts. I will have a look at it later. python 2.7 and older 3.2 versions are not supported anymore, so I will update the documentation. The only limitation I have is that some people use PCSE together with IronPython which is now available for python 3.4 (was 2.7). Nevertheless, I need to remove a lot of stuff from PCSE anyway before it runs under IronPython.

[burggraaff]

@ajwdewit I have resolved the conflicts and it should merge properly now.