Throws error on visual acuity scores that are not a multiple of 5

[dcronkite]

These tests will show the error:

    	def test_logmar_16
		a = VAExtractor.new
		val, _ = a.logmar([20, 16, '', '', '', '', ''])
		assert(val != nil)
	end

	def test_logmar_19minus
		a = VAExtractor.new
		val, _ = a.logmar(["20", "19", "-", nil, nil, nil, nil, nil, nil, nil])
		assert(val != nil)
	end

This will cause the manual variable to be nil:

vaextractor/lib/vaextractor.rb

Line 64 in d7c7261

manual = @usedsnellen[va[1].to_i]

And will cause an error here since lva[0] cannot be compared to bcva['O?'][1].

vaextractor/lib/vaextractor.rb

Line 254 in d7c7261

if bcva["OS"] == nil or (bcva["OS"][0] <= priority and lva[0] < bcva["OS"][1])

I bypassed the error by supplying next if lva[0].nil?, but this just ignores those values.