Add mathematical functions

[ForNeVeR]

I have the feature request from Russian F# StackOverflow chat: add arctg and arcctg functions. I think that's time to review the functions we have and add those trigonometric functions we still haven't.

Please note that the ordinary notation for tangent is tan, but tg is also a common extension I think.

Probably these should go into PredefinedTexFormulas.xml.

[alexreg]

Yes, for sure. At some point it may be advantageous to move from an XML definition to some sort of concise code definition... but I'm not 100% sure yet.

[gsomix]

I have updated issue title. We need to add not only trigonometric functions, e.g. \ln, \log, etc.

[ForNeVeR]

@alexreg now, when I see the whole picture with that PredefinedTexFormulas.xml file, and how you've embedded that small formula DSL directly into the library, that seems like a very smart idea. Take my compliments.

I'm not sure if we want some other formula definition format. What we have seems pretty much what we need, even if XML is a bit "old school". At the moment, I think we'll just extend that DSL and add new features into it.

[alexreg]

Okay, fair enough. I generally like to avoid DSLs, but this one does the job fine for now, as you say. A sort of builder-syntax style of defining formulas would be a slight improvement, but maybe not a priority.