describe() fixes

[Jolanrensen]

Fixes #352 and part of #558

I'll first fix some of the most annoying bugs which can be found with describe(), more changes or fixes will come later.

Fixed:

• created col.isInterComparable() instead of col.isComparable() to avoid confusion
• cumsum:
  • added Short, Byte, BigInteger, Nothing support
  • (Short/Byte are supposed to become Int, similar to sum(), but they are temporarily converted back to Short/Byte so that the DataColumn.cumSum function returns the right type)
  • added extra tests
• mean:
  • added BigInteger support
  • added TODOs for BigDecimal and BigInteger, they're supposed to result in BigDecimal, not Double
• median:
  • added Float, Short, BigInteger support
  • Number support is still missing as it's not Comparable, will be handled later
  • added TODOs for conversion, will happen later
• min/max:
  • removed unused internal code
  • Number support is still missing as it's not Comparable, will be handled later
• std:
  • added BigInteger, Number support
  • added test for Byte
  • BigDecimal and BigInteger are supposed to result in BigDecimal, not Double, will be handled later
• sum:
  • added Float, Short, Byte, BigInteger, Number, Nothing support
  • added some tests
  • conversions will be handled later
• describe:
  • fixed @DataSchema type being String instead of KType
  • some cleaning up in the implementation
  • min, median, and max now also show up for Number columns with different types :) Numbers will be converted to double or BigDecimal first so they can be compared.