directory names with multiple words should use hyphens to separate words

[jbphet]

The PhET convention for directory names that have multiple words is to use hyphens as a separator to make them more readable. With this in mind, several directories should be renamed:

• manualgeneexpresion should be manual-gene-expression
• mrnaproduction should be mrna-production
• multiplecells should be multiple-cells

[aadish]

fixed closing