Add number_of_returns option to FauxReader

This option tells the FauxReader to write ReturnNumber and
NumberOfReturns to the buffer. NumberOfReturns will be the value passed
in via `number_of_returns` option. ReturnNumber will increase
monotonically from 1 to `number_of_returns`, then reset to 1 and repeat
as necessary.

This patch enables easy testing of high return counts.
Skip, don't fail, when writing high return num las

This patch adds a new option to the las writer,
`discard_high_return_numbers`. This is a boolean option that defaults to
false. If this value is true, the las writer will check the return
number and return counts on incoming points. If the return number is
above the LAS maximum of five returns, then the point is dropped.
Additionally, the numberOfReturns is clipped to five.

This is to support data streams that might have more than five returns
for a given pulse.
Merge pull request #413 from gadomski/las-skip-high-return-numbers

Optionally skip high return numbers in the las writer