flaky tests in MatCheck

[pityka]

Elementwise matrix operations with scalar (I,D) => B
[info]   + op < works
[error]   x op <= works
[error]    Falsified after 12 passed tests.
[error]    > ARG_0: [100 x 100]
[error]     1269956579 -1651588045  -23183[90](https://github.com/pityka/saddle/runs/5309342893?check_suite_focus=true#step:5:90)47  1009904595  ...   598167718 -10[91](https://github.com/pityka/saddle/runs/5309342893?check_suite_focus=true#step:5:91)060996  -56654787   376489325 
[error]     -808139313  1961476986 -1414633998  -383026468  ...  1799829621  1321614098 1435773124   358693408 
[error]     -118862603  -518816064   385642582   -17791247  ...  -655062533 -1228565848 1002622225  2070131445 
[error]    -1099712990   639182453  -786264453   205137673  ...  1188727935   539641967 1972840830  1311770901 
[error]    ...
[error]     -297240988 -1704265288  1080866841  2115918235  ...  -179810603   873080620 -964765650  1945350146 
[error]     1566446872 -1128376140 -1901055413 -1774102629  ... -2143441516   -65718677  166885557 -1808064785 
[error]     1555259345   581502718   614097437 -1372645149  ... -1596531520   -78749081 -584278297  -534540360 
[error]     -304506615 -1694821768 -170322[92](https://github.com/pityka/saddle/runs/5309342893?check_suite_focus=true#step:5:92)01   -57132164  ...  1995229777  1356349720 1926262097  21358187[93](https://github.com/pityka/saddle/runs/5309342893?check_suite_focus=true#step:5:93) 
[error]    
[error]    > ARG_1: -5.3055700234605[99](https://github.com/pityka/saddle/runs/5309342893?check_suite_focus=true#step:5:99)E-165
[error]    The seed is rHuHhy2VdhK_RTuBaXb7PkeY6mOGYXBmj_fsLhY-L0J=
[error]    
[error]    > [[100](https://github.com/pityka/saddle/runs/5309342893?check_suite_focus=true#step:5:100) x 100]
[error]    false  true  true false  ... false  true  true false 
[error]     true false  true  true  ... false false false false 
[error]     true  true false  true  ...  true  true false false 
[error]     true false  true false  ... false false false false 
[error]    ...
[error]     true  true false false  ...  true false  true false 
[error]    false  true  true  true  ...  true  true false  true 
[error]    false false false  true  ...  true  true  true  true 
[error]     true  true  true  true  ... false false false false 
[error]     != [100 x 100]
[error]    false  true  true false  ... false  true  true false 
[error]     true false  true  true  ... false false false false 
[error]     true  true false  true  ...  true  true false false 
[error]     true false  true false  ... false false false false 
[error]    ...
[error]     true  true false false  ...  true false  true false 
[error]    false  true  true  true  ...  true  true false  true 
[error]    false false false  true  ...  true  true  true  true 
[error]     true  true  true  true  ... false false false false  (MatCheck.scala:78)

[pityka]

The problem may be in the handling of missing values.
It looks like the implementation in BinOp is not doing missingness check, while the implementation of Mat.map does.