Use np.repeat where possible.
doc: Fix many spelling issues.

Most of these were found with the "misspellings" tools. The remaining
were found using the sphinxcontrib.spelling extension.
pep8: Fix block comment start.
pep8: Fix under-indented examples.
doc: Add some types to segy.

I only moved the explicit types from the descriptions to the sphinx type
directive.
doc: Fix font of math subscripts in example.
doc: Change double to float.