Add functions converting DataType to int/float/str

[Geo-W]

This pr tries to address two issues.

1. Users might mistakenly store number as text in a workbook, this gives the flexibility to read the data.
2. xlsx reader always return float when dealing with number, but in reality not all the numbers contain decimal spaces. https://github.com/tafia/calamine/blob/51116dcba4becb126e29e03b4cf8afcbffb7324d/src/xlsx.rs#L968-L983

[Geo-W]

update function name and recover lines formatted by IDE formatter lol

[tafia]

There seem to be some fmt issue. Can you double check? Thanks