Update mats1 card

[benvb-97]

Hi Steve,

My apologies, I was not able to reopen the previously closed pull request with the same name. Here's an overview of the changes:

• Added card format to the description of the MATS1 class.
• The MATS1 card has two fields LIMIT1 and LIMIT2 that can both be real-valued or blank. The code has been updated to reflect this (previously only allowed real-valued fields).
• Added the STRMEAS field to MATS1.

Update since the last pull request:

• Added backwards compatibility for cards without a STRMEAS field. If the field is blank it won't be written to the .op2 file, and the .op2 reader can handle files with/without STRMEAS entry. I hope this is what you had in mind previously. I'm happy to assist if needed/possible. Thanks again for all the wonderful work you're doing with pyNastran!

Passed all test_materials.py unit tests.

[SteveDoyle2]

looks good!