Added functionality to Split

[falconstryker]

MarsVars split function now accepts single values to pull out one lat, lon, lev, or time.

[pep8speaks]

Hello @falconstryker! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file bin/MarsFiles.py:

Line 507:80: E501 line too long (88 > 79 characters)
Line 512:80: E501 line too long (81 > 79 characters)
Line 513:17: E128 continuation line under-indented for visual indent
Line 514:17: E128 continuation line under-indented for visual indent
Line 516:1: W293 blank line contains whitespace
Line 528:80: E501 line too long (82 > 79 characters)
Line 532:80: E501 line too long (83 > 79 characters)
Line 537:80: E501 line too long (83 > 79 characters)
Line 541:80: E501 line too long (83 > 79 characters)
Line 544:80: E501 line too long (83 > 79 characters)
Line 545:80: E501 line too long (80 > 79 characters)